Look for one with lots of sliders to wiggle and possibly a breath controller input if you want lots of control. Fatar and Roland are two companies that make a range of controller keyboards; Peavey may still - I\'m not sure.
I don\'t know the answer, but you should check out the difference in cost between a synth and a controller keyboard with the same or an equivalent mechanism. It may be too close to buy the controller; the market for controller keyboards is considerably smaller.
Right now I use Kurzweil K2500X as my weighted controller. It uses a Fatar mechanism, it has lots of sliders, and the action isn\'t too horrible. The synth always seems to have something close to the one sound you\'re looking for. You can find them for about $1500 used.
But I use several controllers for different things: an Akai EWI, a drumKAT set, occasionally drum machine pads, and both weighted (the K2500X) and unweighted keyboards. And a breath controller.
I love my 2500X for the sliders, breath control and piano action, but also frequently use my Roland XP80 for synth-type control (almost everything that isn\'t a piano plays smoother for me on a synth keyboard.)
I agree with Nick [img]images/icons/smile.gif[/img]
The more knobs and sliders, the more on-the-fly midi control you have. I use a QS8 at home, which can make do with 4 programmable sliders, and I use a Triton here at work, which has 4 knobs in addition to a ribbon slide. musiciansfriend.com good man, musiciansfriend.com. Marvel in it\'s wonderous bounty!