Hi Chil,
I have just purchased a Korg PA50 recently to add to my PSR2100. I can give yu some of my inputs regarding these 2 boards in accordance to your queries.

(1) Keyfeel:

Hands down Korg...the PSR2100 keys now feel inferior after I played on the korg. Korg keys have a good springing action which the Yamaha lacks. Sensitivity is more or less same on both..Korg is slightttly better.

(2) Creating new rhythms on 7/8, 5/8

This also goes in favour of the Korg. Korg gives you good flexibility in terms of creation of new rhythm and also fully altering the in-built factory styles. Although the navigation thru the KORG's OS is more of a quest than anything else. Yamaha is more user-friendly OSwise.

(3) Sequencing Operation:

For ease of use with sequencing, I would go with Yamaha, but for Sequencing power, it's KORG all the way. Then again, U have the cumbersome Korg OS to deal with.

(4) GM-GM2, XG, GS support

I found both keyboards support these formats quite well. Yamaha would score a little more here.

So, there you are. Im sorry I have never played on a Roland EXR-7 but you have some ideas about the other 2 boards. Also, the Yamaha styles are smoother, CD-like. The KORG styles are a bit on the rougher side...a bit busy sometimes. I mix and match what I like and don't on both the boards. But the 2 boards together make for a great pallette of sounds and styles. All the best in your decision.

Cheers
SUNNY

[This message has been edited by sunster (edited 11-17-2005).]