| are you a keyboard player or a bass palyer?
no need to use your left hand for synth bass first of all if you're a bass player.
I use my laptop running Reaktor by native instruments with a USB M-Audio keyboard. Really small rig, easy to take on the road, and some incredible sounds.
I've also used the Nord Lead 2, and some Roland sound modules with various midi keyboards.
The thing with the laptop is that the programming is so much easier for me. I'm more of a digital guy when it comes to synth bass than an analog cat. Then there are various interfaces you can use to send the sound to the front of house. I've often just used the mini jack output with a mini jack to a Stereo Quarter inch pair into a pair of DI boxes. It's worked great for me so far.
It really depends on the gig and on how much gear the artist or management will backline for you. I've had them get me a Yamaha motif 6, the korg triton, Roland Phantom just cos it's been a one off and I didn't want to bring the stuff with me on the plane, and those keyboards are pretty easy to program too.
Easy,
Janek | 
