As mentioned by others, it comes down to what you want to do with it and how much you want to spend. I debated for a long time between the Uzi and the Sterling. As noted, the Sterling is so much smoother. I ended up getting the Uzi, and have been happy with it.
For competitions, I go back and forth between the Uzi and the M11/9. I bought the M11 back in the day for subgun matches, and immediately did the TASK slowfire conversion to it. Later came the various Lage uppers. I've done well with any of the setups.
There are the rifle-caliber uppers out there now, and the Lage upper on the horizon. I find it somewhat ironic in that I always wanted an M16, but would likely shoot it in 9mm 95% of the time.
As for being able to customize the Uzi, besides the standard open bolt 9mm, I have setups in .22, .357 Sig, .40S&W, .45 ACP, and closed bolt 9mm. But I shoot it almost exclusively in 9mm open bolt.
If you want to run suppressed, the Uzi seems quieter to me with any can used. Also, the M11 tends to blow a lot of gas in the face when running suppressed, where I've never had an issue with the Uzi in this regard.
If I had neither and was shopping for one, I would buy the M11/9 with Lage upper(s). The two feel pretty similar. The TASK setup is smoother, but it seems I'm constantly having to tinker with that one to keep it running right.
I occasionally consider selling the Uzi and Reising and buying a converted AR15 instead. And then I usually remind myself to be happy with what I already have.