You can't compare these.
Personally I think you should own both.

The K2600 has on board sounds and with the extra rom boards you have a pretty nice
set of sounds for take off, but afterall it is a synth to create your own sounds.

The roland has many more standard sounds ready to go which is very handy.
Sounds are "thinner" which is not allways
a disadvantage.

In quality (specially the dynamics/depth) the K 2600 is unbeatable and gives much more real instrument feel.
But this is not allways what you want.
F.i. in a mix where you need background orchestra a bit less dynamics would be preferred to avoid overuling the solo instruments.
Ofcourse you can tweak the sounds without a problem and just store in a bank, so it is not a problem, but sometimes it takes some time and I allways loose inspiration as soon as I have to tweak sounds.

Then ofcourse there is KDFX on the K 2600.
In my own experience one of the best effect studio's you can have.
The most important difference here is that when you record a track it is "ready" does not need any compressor or other wave editing, it is just ready.
You just master your tracks and that's it.
I even use the KDFX now for other gear as an effect studio.

Any other synth device I own, allway needs some tweaking/wave editing once recorded.

If you are planning a K2600R make sure you have the sampler option installed.
You need the sampler option installed to use KDFX for other gear.

Fred