I have both keyboards, and I find the DGX action very good actually. However, the action of the DGX is loud, comparable to that of an acoustic, but the MP6 action is very quiet. The MP6 has tons of features that you may or may not use, such as layering multiple instruments.
The MP6 is a much more solid build, but has no speakers. However, it has a line out + phones out, while the DGX only has phone out.
The DGX has a fitted stand, while you'll need to invest in a decent stand for the mp6. Dont get a cheap X stand. Get a nice table stand.
The DGX has a joke of a pedal, so you'll want to buy a better one for $30 or so.
The DGX has a low quality components and will break after a couple of years. The screen will give up on you.
The MP6 has tons of effect to manipulate the sound of the piano to your liking.
The MP6 is the best looking keyboard out there, in my opinion. It does have rhythm accompaniement but it's kind of hard to get to. On the DGX, a child can do it. You can plug in USB and play along to MP3s, but again, kind of too hard to control, so not really valid.
Keep in mind that unless you plan on playing exclusively with headphones, you'll need a solid set of speakers for the MP6, that will run a few hundred dollars. What's inside the DGX is no high end speakers, but it's decent and it's hard to match on your own.
One of the most critical aspects of learning on a DP is matching the volume of an acoustic. Playing way below acoustic level will hurt you more than anything else and will give you bad habits.
The MP6 is very much a stage instruments. For home, it'd take a serious look at the Roland FP-7F, if you're in that price range. With speakers, should be about equal to the MP6.
