I decided on the RD700NX after reading many posts here and other forums, and watching many, many reviews and demos on youtube. I research my purchases to death lol.
I was considering mainly the RD700 and the Yamaha CP5, but I was also interested in the digital uprights from Roland and Yamaha. Such as the Roland HP-505, HP-507 and LX-15. And Yamahas range of digital uprights (I think the range is called CLP). And I also considered the Yamaha Motif XF-8, the Roland Jupiter and the Kronos.
My main factor to consider while choosing was the action. How realistic the feel of these digital pianos are compared to an acoustic piano. Reason being, I won't need to perform with the instrument for a long time, so the feel of the keybed was much more important than the sound it produced. My thinking was that the closer to an acoustic the action is, the better my technique will be able to develop and become. And that is all I cared about for this purchase.
So in all my research, in all the posts and comparisons between the models, the RD700NX was said to have the closest action to "the real thing". I don't think I heard a single argument to the contrary. And it is very good. I am very happy with it.
I played quite a few instruments - most of the Roland line including the V-Piano and the big V Grand and quite a few acoustic pianos (just to compare). My opinion is that Roland's digital pianos like the RD-700 have a similar feel to some very expensive grand pianos. The cheaper acoustic pianos ($7-25,000) felt VERY different (I didn't like the feel at all but I think I would get used to it when I eventually get an acoustic).
The Motif, Jupiter and Kronos were ruled out because they are workstations and you pay for the incredible features of a workstation. The RD700 and CP-5 are dedicated stage pianos. You don't get nearly as many features but they are designed solely to feel and sound like a grand piano. I didn't get a chance to play the CP-5 but I was content that I would chose the RD700 anyway due to what I have read. I believe the CP-5 has an equally weighted action which for me would probably rule it out alone. I also read that it has shallow key depth, but that is also second hand information.
I suggest you play these instruments for yourself and see what you like the feel of. The Roland keys are heavy. I actually tested them using USA coins as weights (left over from a holiday there). The weight required to depress the keys at all was 53 grams at the top and 62 grams at the bottom. At middle C, 45grams is considered light, ~50 grams is considered normal, and 55grams a bit heavy (according to what I found online - please verify). So according to this scale that I found online my RD700 has heavy to very heavy keys. A lot of people believe learning on slightly heavier keys will make you a better piano player. But many many hours on heavy keys can lead to problems. I am no expert on key weight. These are just things I was curious about and read up on.
Finally, I had a problem getting a sound I really liked out of the RD700NX. I tried so hard for days to adjust settings and even downloaded software pianos (hooking the RD700 up to my laptop and using the computer software to play different piano sounds - sounds recorded from real grand pianos). I tried 3 different quality headphones but nothing worked. It sounded muddy or muted. A bit hard to explain. I went back to the shop and talked about it and jaws dropped- I felt like they were going to shoot me for saying that about the RD700 lol. I had already decided that I wanted to change it for the LX-15 upright. They were very very good about it and were happy to exchange it but wanted to send someone out to look at it first. Well I agreed but I listened to and played the LX-15 and HP-507 (HP-507 is cheaper but still more expensive than the RD700) and I was sold on the HP-507. To my ear and the piano teachers ear, the HP-507 sounded as good as the LX-15. I bought the HP-507 and was to decide whether or not to keep the RD700 after someone came to look at it. Btw, the shop owner said that the HP-507 was a "toy" compared to the RD700. But I absolutely disagree!
The person that came and looked at the RD700 wasn't hearing what I was hearing. He made some very small adjustments but to his ear it was fine so there wasn't much he could do. I kept the RD700 anyway because I got it for a price I could sell it for and not take a loss at all, and I figured it HAS to be able to produce the same sound that the HP-507 makes. Well to this day, with headphones on I still think the standard HP-507 concert grand sound sounds much better than the best sound I can get out of the RD700 so far. But here is the weirdest part. I finally got around to recording myself on the RD700 and playing it on the computer and it sounds AMAZING. As good as I could ever hope for. So the problem lies somewhere in the sound getting to the headphones. Not what the RD700 is actually able to produce. Of course this is only a consideration if you plan to use headphones (which most people looking for a digital piano do). Also, it is quite likely that the problem is just my taste and the sound is perfect to most other people. So don't be discouraged at all until you hear it for yourself. There is a good chance you will love it and it is just me that had trouble with the sound.
My advice to you would be to play as many stage pianos as you can. Take headphones with you. Play for as long as you can on each of your narrowed down choices. If you like the sound of the RD700NX and the price is in budget, I HIGHLY recommend it. If you don't need the portability and have the budget for the HP-507, I recommend that over the RD700. I'm sure many people would disagree with me and prefer the RD700. Also consider that if you plan to play your music to anyone you will need speakers for the RD-700, which can be costly for quality monitors. The HP-507 obviously has them built into the piano (and it mimics an acoustic). So factor that in the cost as well as needing a stand and possibly peddles (if you want all 3 - which makes the sustain peddle really stable as well) for the RD700.
IMO the RD700NX is the best current choice for someone wanting to learn piano on a digital stage piano. It is EXCELLENT. The PHA-III ivory feel keys are superb and the best I have ever felt on any digital piano. The action is equally excellent and was the deciding factor for me. Still check out the HP-507 if you can
btw, the HP-507 takes up less space than my RD700 because of the stand that was made for the RD700 (but you can get smaller stands).
This is a very long winded post. Sorry about that but I hope it helps you a little
Ps. The only thing I regret, because I ended up spending so much money anyway, was not checking out the Yamaha Avant Grand N series digital pianos. I think they are VERY expensive. But if your budget allows maybe check them out too.
I should also have mentioned the Roland FP-7F. This is another digital piano you should consider. I believe it is very similar to the RD700NX but has a much lower price tag. I think it's exceptionally good value and it is very highly regarded on these forums. It has the same PHA-III ivory feel keys with escapement! and it has an excellent built in stand that makes it a really neat package taking up less floor real estate. With this stand there are no cross bars for your knees and legs to hit - i learned about those stands the hard way. Yu could spend the savings on some seriously good speakers and many other things you may want when you start playing (one example.. the software available for these pianos is incredible) Definitely check out the FP-7F FP-7F - Click Here!!