I have shot extensively with both the MP5 and the UZI. Personal UZI and employers MP5. Both selective fire. I may catch a little crap but I prefer the UZI over the MP5. I like the fact that the UZI is far more user friendly when and IF something breaks. Try that with MP5. Parts are more plentiful for the UZI and ALOT cheaper. I have fired well over 100,000 rds through MP5's and I do like the way the gun shoots but I would not trade my UZI for a MP5. I have close to 20,000 rds through the UZI never problem or a jam. The UZI suppressor lets you use 147gr ammo whereas the MP5SD is reccommended to use standard 115 or 124gr ammo. The barrel on the SD is ported to bleed off the velocity. The UZI is not ported and you can use the heavier subsonic rounds with the suppressor. I do not use MP5 anymore at work, I was issued HK G36E 4 years ago, but thats another story.
I hope I was not too long winded, but that is my experiences with both of the superb firearms.
Joe