I would get a 125 or 150. I started out on a 50 4stroke Honda and going uphill was a scary proposition as the traffic would back up on me quickly.
I did like the gas mileage I got out of it.
When I lived in MO you didn't have to register a 50 nor tag it. However, many a person asked me if you had to have a license for it. Since a lot of them had lost theirs, this seemed like a good way to cheat the system. Unfortunately, I believe they have to have a valid drivers license to operate a motorized bicycle (which is what they consider anything under 50cc).
On top of this, MO has the stipulation that it can't go over 30 as well (there's something about brakeing horsepower limits but how the heck one measures that, I don't know). I imagine any cop that wanted to could hassle you.
In Kansas, you have to license and tag a 50. Still, no motorcycle endorsement is required.
I see a whole lot of people that get a 50, do all sorts of mods to them (2 strokes), and then can't understand why they overheat and just exhibit a general level of unreliability.
