I've read in another forum That the Yamaha p105 is on the lighter side of average keyweight while the casio is on the heavier side of keyweight.
Apparently keyweight can get Very Very heavy as some people prefer the Heavy keys... more dynamic control I guess.
I am like you.. My hands tire Fast due to a condition..
If I were you i'd go with the P105 as it seems to have the lightest action from what I've read (GHS)
I myself might get one..
If you are now starting back your hand muscles are probably a bit weak.. It should get easier.. Just remember to take frequent breaks and not to overdo it.
IF your RSI is acting up STOP..rest.
Moderation is the Key for RSI injuries..
Mine isn't RSI unfortunately... a bit more complicated but I manage it.
Kawai seems to have a heavier action from what I've read as well.
The higher end yamahas such as the p155(GH action) also are heavier than the P105. But are supposed to be better actions.
Are u sure it isn't the P155 you played?
Again if I were you I would probably go with the P105.
The Best thing is to go in the stores and try them out for yourself. Something I need to do again.
I've played Yamaha GH (action in the P155) but was a long time ago and I can't remember how it felt..
Temperament is right about the casio tho..
It's a tri sensor design so it would probably give you more control over the Yamaha.. But considering your unique situation with the RSI I personally would recommend the Yamaha p105..
Again all that I've said is based on all the research I've done and from what I've read.
Digital Pianos are hard to come by where i'm from. We basically only have access to roland..
I was going to special order a casio but After I've read about the heavier action I will go with the P105..
The Previous PX-x30 series actions are supposed to be lighter than the new PX-x50 series but again, from what I've read the quality wasn't as good..
PX-x30 keys had some complaints of being wobbly? and some quality control issues.
You are lucky that you have the option to to try them before you buy..
And don't worry about notes dropping out as both Yamaha and Casio has 128 note polyphony.