I've been making beats on my crappy Yamaha keyboard for 6 moths and now I' ready to take it to the next level. I 've looked around and the Miko and the MV-800 both seem to have what I'm looking for. But I'm not sure if you can sample with Miko?

Has anyone had any experience with these two machines?
The Miko is just a Windows XP PC with a keyboard attached. It can do anything a modern computer can, as you can run a DAW on it (Sonar, Cubase, Nuendo, etc) to take advantage of. The Roland is proprietary and limited in its function, while the Miko is open ended and can be upgraded and modified extensively both in hardware and software.

That said, what exactly are you looking to do that requires this kind of hardware? Why not just get a higher end Akai production unit, they are the industry standard and miles ahead of Roland and Open Labs in terms of dedicated groove boxes and beat production, as well as sampling.
I'm just looking for something that produces good sound quality and where I can record ,mix,and use samples easily. I like to make music with exotic percussion, Synth sounds, and turntables.If I play guitar I'll use the virtual amps. Akai makes the MPC but I heard that's its losing popularity. And that the MV-8800 is better.
Any of them will work fine, I just think Akai is superior to Roland in this market.