I have fired the Uzi. Never fired the S&W. If you have handled both and like one over the other. Then that should have some bearing on your decision. Another factor which may or may not help.
The S&W mg. Is not well known. Fact is few people know that s&w made a mg.
This may have a effect on it's appreciation value in yes. to come.
Would go with a correctly modified IMI or repaired Vector (six of six in the bunch need some TLC) registered receiver gun, unless you were looking at a mini Uzi option (in the old days did not like the micro but it is like a M11 380 a special purpose sub gun).
Would also look at a Sterling Mk4, lots nicer shooting to many than an Uzi. Had a tube that sold for a good price to a close friend years ago, to get a MP41. He uses it now instead of his Uzi to compete with.
The MK760 is interesting but most need a little adjustment of the bolt/ejection process (i.e. if going that way need to do more research and/or test fire). I got a Sten MkII myself for a "so ugly it looks ok" slow fire 9mm and shoot it more than the Uzi ... but still like the MP5s the best.
Sterling Mk4 is one of the smoother open bolt guns with a moderate rate of fire. The MP40/MP41 and Grease Gun (M3) is so much slower that they are a different animal.
Be patient and keep your eyes peeled for an Uzi. You can find a nice registered receiver gun for $7K or less. Be ready with cash and don't flinch when an opportunity arises. The Uzi is an outstanding first SMG, easy to load, easy to run and lots of neat accessories. You won't be disappointed in one even at premium prices. With many of the others you will want something more later.