Well, wellk, well, did you ever luck out, becuse I have both a semi Vector Uzi AND a 9mm Rock River AR with the VM Hytech block, and can compare the two!
The Vector Uzi and the 9mm AR are apples and oranges., here is my take on each:
The Uzi is a great "fun gun" although a bit heavy for what it shoots. I definitely like ripping through mags on semi auto, but it is nothing like an AR-15. It is also 100% reliable and runs right out of the box. this is also the easiest 9mm longarm in the world to clean, thanks to the removable topcover.
The 9mm AR-15 is definitely a "frankenstien" beast, and may take a little tweaking to get running, but when it runs, boy does it run! The ergonomics are better on the AR-15 and it seems (at least for me) that the accuracy is better. Accuracy in part may be due to me having a lot of trigger time on AR's vs. the Uzi (three years vs. 6 months for the Uzi). The 9mm AR is invaluable if you want to keep the same generla "Manual of arms" and keeping up your skills with the AR during the winter.
One thing I was concerened about was the VM Hytech blcok and the change in magazine catch position. A lto of people reported they would accidentally press the .223 magazine catch and dump the block on the ground. What I have found is that having used those Uzi stick mags makes me hold them and insert them just like the Uzi, so I have never any problems with accidentally "dumping the block." In addition, at least on my AR the block fits snugly enough aginst the upper when inserted that I can't depress the .223 mag catch and acccidentally dump it- I actaully have to split the upper/lower to insert it and lock it in, which is a good thing IMHO.
The 9mm upper + VM Hytech mag block + Uzi mags may require a slight amount of tweaking to the Uzi mag followers to get all the rounds to feed. Due to the steep angle of the last couple of rounds on the uzi mag follower, the 9mm AR bolt doesn't always strip the last 1-4 rounds in about 30-40% of setups. the easy solution is to bend the legs on the follower a little until the angle flattens out enough that the mag will feed all the rounds, and yes, the mags will still work fine in your Uzi.
I would highly reocmmend having both setups, especially if you want to practice with your AR skills on a pistol range- that is exaclty why I got my 9mm AR upper after getting the Vector Uzi. Feel free to email me at dawg180[at]yahoo.com if you have any other questions. I post a lot on the AR15.com pistol/rimfire caliber forums and helped set up the FAQ over there, so I have a pretty good idea what is going on.
Dawg
IMHO, unless that colt comes with several (5+) colt style mags skip on it and go with the RRA setup. You are simply paying for name brand and from what I read you want a shooter, not a collectors piece.