I've shot a fair amount of ammo through both (full-auto). They really reflect two different philosophies on subgun design (550rpm, open-bolt, mag in pistol grip vs. 800rpm, closed-bolt, magwell).
Both are classic firearms. The Uzi is probably the winner from a maintenance and reliability standpoint; but in every other point I like the MP5 better (ergos, accuracy, controllability). As long as nothing on the MP5 requires it being sent back to H&K or getting a part from H&K, it runs just fine. Both are more than adequate for the role they are designed to fufill.