I don't know if anyone can give the right answer on that Claudiu. I know I can't. It's really going to depend a lot on the type of sounds you need. My best suggestion is find a Motif to demo and try it out.

I have only played a little on a VA76 quite a while ago, so I cannot remember much now about the sounds. As far as the WK goes.. I'm getting so tempted to buy one to hack around with. I had an MZ2000, which was Casio's one attempt at producing a higher end arranger. The WK 3700 supposedly uses the same sample engine. I liked some of the sounds a lot and the synth editing engine was about as good as any that I've ever used in any sample based machine. I disliked many of the styles though.

If the board you choose is mainly for playing solo voices, then either version of the Motif is going to have nice sounds. In a band setting though, in my own situation, I sometimes prefer to use my laptop with softsynths ( for even better quality of voices ) even though my ES is always there, since it also doubles as my midi controller.

AJ