If you want more realtime controls and something that's fun to use live, go with the MC-909. The MPC is better for production overall, but it costs a lot more. I've also heard that the MPC4000 has one of the steepest learning curves amongst MPC's.
But to be honest, if you don't know what you're doing and you're just getting into production, don't throw down a huge chunk of money on something unless you know you'll be able to take advantage of it. If you just wanna get into making beats and have fun, get a Korg Electribe... one of the sampler series.
I've never used a MV... probably never will..