Vagro,
This is really a tough one...
I have a T2 and a PA2XPRO. Both are excellant. Both are also very different.
I kind of see it this way. (not a complete list by far)
Remember I have the T2 and PA2 so that is my experience here. Everything I talk about may not be on the models you are looking at. But in general I think will be applicable.
Yamaha:
Excellant sound overall, all the sounds work nicely together. LOTS of styles available. SA voice are good, but some, including me find a learning curve and also some seem to be a little light in sound unless you play then real stacato (guitars). Sax is super , horns are cool with some nice articulations and expressions. Piano is very good. Yamaha does not make all that much use of aftertouch so most of the other sounds don't really use heavy aftertouch. I don't think the s900 has aftertouch anyways which is a show stopper to me, but the T2 does. I love aftertouch on vibrato and for building sound loudness as you press (this is great on orchestrial sounds). The Yamaha songbook feature is sorely lacking function. Hope they fix that in T3.
To control external sound module with the T2 (assume s900 is same) leaves a lot to be desired for sure. You can't program your own bank/Prohram Change data on a user voice. And you can't set a user voice to external only. I don't think Yamaha thinks that is important and they are wrong. A lot of us like to add sounds to the arranger (modules, VSTi's etc)
If you don't care about that stuff then no big deal.
The sound is excallant but not 'in your face live' like the Korg is. This is not necessarly a negative as some like that.
Yamaha has a very refined arranger function for playing the styles.
T2 can't record MP3 (wav). Sequencer is very poor for detailed editing (message level).
The Yamaha is a very complete, great sounding arranger right out of the box. If you can't find a style for a song you want to play...you haven't looked! I like that.
Korg:
Great sounds overall too. Great styles on oard, some others available but not like the Yamaha has. Also,user third party utlity programs are slim to none, but that will change in the future (Korg is getting bloodyed over this as we speak). SUPERB Piano in the PA2, not sure about the PA500.
The RX voices are very nice and do have some articulation features but it is done differently, more with velocity, aftertouch and mod wheel. I like some voices much better on the Korg and vice versa! I think the Korg is a little better for more mod music (IMHO) just because it is in your face live more. Drums are great. Percussion is great.
As to flexibilty in programing options and functions the Korg has way more. This I like, but some may not need it.
You can easily control the program selections on external sound modules. Superb feature.
Great style creation, full editing PRO sequencer, Full function songbook, dual players, record to MP3 (not sure about PA500)
THe sound engine is a full generation from the TR. It is the same basic one used in the PRO M3.
Don't know if the PA500 has aftertouch, that would be big fo me. But maybe you don't want/need it?
The Korg is great out of the box and is probably more capable of adding to, enhancing it yourself as I think it has better built in tools...BUT the Yamaha has a lot f great aftermarket tools that you can use on a PC.
I said this would be tough... Now I am thinking I left a lot out for each???
Wish you could play both, each for at least a day beacuas all this means nothing if you just like one over the other!
I would suggest you go to the mfg sites and listen to the demos...AND download and read each ones manual. And I mean really READ them. That will give you some good info for sure.
Don't know if I helped or not, hope so.
Maybe now it might be obvious why I have both? Can't decide. Wish I could blend them together! :-)
Lee
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Lee S.