I would probably go the used Kymco (trying to do better on price) over the Chinese 150cc. There are some things that factor into going 55mph a lot and wanting to sometimes go faster (on an expressway). The riders weight is a factor. I started on a Honda SH150i (150cc) and it will do 55mph in most circumstances, once I get it there (not up a steep hill). I am, er, "fluffier" at this stage of my life than earlier. If I weighed 130 lbs, a 150cc would probably get me to 60-65MPH and keep me there in most circumstances. I can get the Honda 150cc up to 60MPH on a level, and usually maintain 55MPH unless the grade is long and steep if I'm already up to 55-60MPH when I hit it. I have not owned a 200cc, but I'd be betting that extra 50cc helps a lot (I've seen someone with the Kymco DT200i say they top out around 70MPH, weight of the rider, accessories like top cases, etc. being an unknown). If you are riding at 55MPH *a lot*, unless you're a fairly small (light) person, it'd be my gut feeling that you'd be happier with 200cc. Brand-wise Kymco is going to be better built and have a better service network than a generic Chinese scooter, generally.
That said, if financing is an option, I'd probably be looking at spending a bit more, using the current 1.9% (in the US), 0 down (they may want a deposit up front if they have to order it), 3 year financing and going new (some places you can knock some off the sticker, at the local Kymco dealer here, it seems to be about 10% or a little better on the higher end, I have not priced Kymco stuff on the lower end in 150 or 200cc). That also would get you the 2 year warranty if you have problems. (Like 200i or Like 200i LX, I guess are the current offering, if your local dealer has an older year around the showroom, there might be more room to deal as well). But, of course, none of us knows your exact circumstances.
I just got a new DT300i and skipped the 200cc, after having dealt with 150cc (which is still good for some things and I will continue to use it). One of my reasons was to be able to travel on expressways for longer trips (here, the the current speed limit on limited access 4-lane is 70MPH, and they are going to increase it on some to 75MPH, so I wanted to be able to comfortably do that, as well as to be able to allow for a passenger).