Just go for any of the two. In every-day usage they should both be up to the task. I'm not aware of any Amiga games especially needing the 3D part of the Cybervision, but it won't hurt having it, just in case ;-)
The scandoubler/flicker-fixer of the PicassoIV works brilliant, but not neccesarily handle all the modes the way you might expect. I remember having the card installed in a A4000T and still having to have an Amiga-monitor attached to the RGB-port in order to play Pinball Illusions (somehow the Picasso output went black when playing that game).
So, for the modes failing on the Cybervision or Picasso you still might need a monitor-switch to switch between the 3000T's VGA output and the Cybervision / Picasso output.
