The nice thing about modelling is that it's lightwieght and generally pretty cheap. You can go to most clubs and DI strait into their board, or plug into a home stereo, or a power PA.....so they're pretty flexible. They're great if you need access to a large number of sounds and mediocre quality isn't an issue.
A valve amp on the other hand has tone. They're heavier, more expensive, require maintinance, you have to drag around a cab with it, and you (generally) have more sound issues due to the need to mic. That said, a valve amp has tone, is more touch responsive, and a greater range of sounds within one sound (if that makes sense). Where they really shine IMO is in their dynamics......models, no matter how good tend to sound somewhat flat. Valve amps on the other hand (when turned up) have an immediacy that'll kick you in the balls.