I would definitely get the M16.
Regarding SMG's:
While you have the UZI and M11/9, they are really best as open bolt SMG's. Yes, I have closed bolt conversions for the UZI but the window to tune them is pretty limited and I don't think they run that smooth.
If you ever get into SMG competitions, you can shoot your UZI or M11/9 in the open bolt class and run the M16 in the closed bolt class.
However, I think the standard blowback 9mm M16 configurations suck in FA.
Fortunately, we have the new radially delayed blowback CMMG Guard that really smooths it out and I no longer have a desire to dump money into an MP5.
On that note, FM Products is coming out with a new mechanism which is probably (hopefully) also some kind of delayed blowback.
I also know of another MFG that will be releasing a drop on piston upper for the M16/AR15.
So much support and future developments going on for the AR/M16 family.
Regarding Beltfeds:
Some mentioned the Shrike/MCR which I have and runs great.
Another option is the Freedom Ordnance 9mm belt fed which I also have and also runs great.
Freedom Ordnance also mentioned the possibility of coming out with a 308 belt fed some time ago so who knows.
22LR:
I love FA 22LR and with the UZI and M11/9 you only have around 30 rounds max. Fun is just getting started at 30 rounds.The Black Dog 50 rounders run great in my M16 22LR conversions.
I also have the belt fed Lakeside LM7 which mechanically works identically to the 1919. No longer made but there is a discussion of a company selling them but I suspect it may be leftover LM7/Razorback parts vs new builds but who knows: https://www.ar15.com/forums/Armory/LithocoreX/23-491320/Again since it works just like a 1919, if you are also interested in the 1919 not just for being a belt fed but like understanding the mechanics, the LM7/Razorback may help scratch that itch to get a 1919. I know it does for me....I get to enjoy having basically a mini 1919 plus:
1. shoot the cheapest ammo available
2. easily shoulderable
3. suppresses well
4. don't have a drag a tripod to range
So that is 3 belt fed options for the M16.
I think it also comes down to the type of person you are and if you are more into collecting than shooting. Even with that point, if you don't have an M16, you should get one now. Get the 1919 later if you really want more MG's vs more conversions.