I own an MP5, and have fired many an Uzi. I pretty much agree with what Batholomew said. Uzi is darn near indestructible. MP5 is very reliable, but you'd have to give the Uzi a slight edge there. The MP5 is much more accurate and comfortable shooting.
If I were by myself and engaging targets only in CQB, I'd take the Uzi. If I were shooting longer distance, especially if I were not alone, I'd take the MP5. Reliability is not quite as much of a concern if you have partner(s) to help if your primary weapon goes down.