I see you wrote "lightweight synth". If it's a good lightweight synth you're looking for I suggest you have a good look at the new Roland Juno Di.

128 note poly
Fantom G sounds
**has that great Roland 88 Stage Grand Piano
USB File/Audio player
Mic input with effects AND vocoder (mic effects are even independent from the tone generator)
Very logical layout
FULL SOFTWARE EDITOR that edits parameters not available on the synth (puts patch editing at a more pro level with the sofware)
VERY LIGHTWEIGHT and (drum roll) Roland drops a Juno that runs on freakin batteries!

It's selling for $699 (not a bad price considering the poly, patch set, USB player, and mic input with vocoder.)