I have the Tyros2 and the Pa800, there are wonderful keyboards, so, are the ones you are considering.
I also came from an organ background(Hammond X66, B3, Yamaha Electones, etc).
It is possible to use the PK5, PK7 Roland or any midi pedal board for bass. (I did use the Studiologic/Fatar MP17.
You Just need to assign the proper midi channel in either of your choices.
The RX technology Korg came out when the PA1X/PA1X Pro where unveil, at the time the Tyros1 had only the Mega sounds that are suitable for style or midi file playing, also had the Live, Cool and Sweet sounds, but to MY ears, THEN, the Korg sounded better.
When the Tyros2 came out, and I play with the new S.A. I knew that I will have to have one, and I did, and I am very happy with it.
To me, RX sounds are more like the S.A. sounds, because you can play live with those, you can not with the "Mega" sounds in the Yamaha.
Only You can weight the Pros and Cons upon the desired use of it. THere are LOTS more styles available for the Yamaha than the Korg, yes, will take more time to get those...also is VERY different the way you load them, there are NOT individual items, in the Korg there are group together in a User Bank that may contain 8-16 or more styles,
I know it for my own experience having both brands. Also the learning curve in the Korg it may take sometime to get used to, even more if you are already familiar with the PSR user interface. ALL the user interfaces can be EASY when you KNOW them, is just matter of time and the effort you put into it.
THe Tone generation on the PA500 it contains many RX sounds of the PA800, besides the TR sounds that you are familiar.
Some features that are NOT available in one or the other are:
Score view in the PSR if you think you need it. This feature is NOT available in the Korg.
If playing a midi file in the PSR, if you don't like the drum track, you just select the style that has the drum pattern you may want, and just press start in the Arranger section and it will be Synch, the benefit of this is also you can put the Fills, breaks change variation, etc.
Since the PA500 is a tad smaller than the 800 and I do not have one, I did notice when I went to the store to see it, it does have a LARGER screen than the 800, larger Icons, only that is monochromatic, not in colors. But is EASIER to see it.
So, depends if you plan to use in the future USER styles, there is also the PSR S900 has the Audio recording feature that is not available in the PA500, and in the PA800
only if you add the MP3 encoder option card that is around $250.00 dls. or perhaps a little more since Korg did announce an increase in the price in the PA800 and PA2X. since these models are made in Italy, I do not know if this will affect the PA500 since is made in China.
I also have as backup the S900, to bad you are so far, I am selling it, I do not use it at all.
Che !!, the PA800 has a wonderful New Tango, it actually has several. THe Korg is excellent for Pop and contemporary music too, it has a little more "punch" (live), than the Yamaha, since is a more polish sound, more like a CD.
Good luck in your decision, for your peace of mind, BOTH units are Incredible, so there is not a such thing as a bad choice. ;o)