Type A is a good speedcube, and the most common. Yes buy.
Type B is no-no. It loosens up after lots of use, and it has the worst pops.
Type C is not so good.
Type D used to be godly, but now, they have changed it and it is the worst type of all 6.
Type E is good, it locks up occasionally, but overall good.
Type F is about as good as Type A.
Get Type A, or Type F. If neither of those are available, get E.