You could consider more options than these two ones.
The Karma is basically a Triton, with an arpeggiator section designed by stephen key.
A rompler wich has programmable knobs for the arp features, buttons to store chords, and it has some nice string sounds. 16 tracks linear sequencing.
About the f**k arppegiator, one of the most complicated and annoying arpeggiators I have used. It is a huge section, almost as big as all the rest of directories. It has "4 modules" wich are nothing else than tracks. There are around 1500 types of pre made sequences, wich can adopt multiple forms.
When you use the 4 modules, the synth becomes really slow. Several seconds to abandon or enter any screen, lots of menu diving makes the experience even more annoying.
The poly is 64, and it easily collapses with too much effects on the arp section.
Actually the arppegiator is a good idea, but with a terrible implementation inside a synth that seems to be not designed to manipulate it. You spend more time programming an arpeggio, than creating a whole sequence.
If you buy a Karma and you want to use the software editor, you must pay $30 to Stephen Key. Yes. Each time a karma changes of owner, you have to send a mail to this guy, and pay him royalties for installing his software on your computer.
I have used and owned lots of romplers and workstations, and the karma was one of the most boring and annoying experiences. CAN´T recommend it.
About the CS6X, don´t have too much information, I owned a Motif, EX5, W7 and SY99, but never used a CS6X.
Do you need specifically a rompler? One of the best ones with keyboard, is the MOTIF. IMO is the best one if you look into Fantom/Triton/Motif area. BTW I also had a Fantom (first version) and it goes second after the karma....