I believe those were just Mini-14's, not AC556's.
It was between an UZI and and AC556 for me too - that's what I boiled it down to. At the time the AC556 was fully factory supported - that was a factor in the decision (I'm not too pleased at Ruger for not at least producing a few parts to keep these guns in service - makes no sense to me, they got a lot of good PR from taking care of their older guns.)
The thing is, if you choose and UZI that is basically a registered bolt or "slotted-bolt gun", then you are relying on that bolt never wearing down or breaking. Now, granted, I've never heard of one of the registered bolts breaking. And, then there were different ways to make the bolts a full-auto conversion: most were slotted FA bolts, but some were SA that had a lower lip added to the bolt face. If I were to choose an UZI, I'd want one of the registered receiver guns - much easier to replace parts.
In my view, you need to decide if you want a sub-gun or a gun that fires a rifle cartridge first. If you decide to go the sub-gun route, I'd try to stick with 9mm - cheaper and more available. I believe that most of the FA Mac10's are 45's. Most of the FA UZI's are 9mm. A registered receiver UZI is going to be expensive (heck anything full-auto is going to be expensive).
I'm glad I chose the AC556 - but when I purchased it, it only cost me $2,400. Some parts on the AC556 are interchangeable with the Mini-14, and if I need a new barrel, I can have one made by accuracy systems, so it is serviceable.
The FNC is another option. You can probably find one for about $7,000+/-, which isn't bad for a machinegun. Parts are difficult to find, but these conversions have an excelent track record and are very sturdy guns.