I think you sort of hit the nail on the head. The Chinese mills (X2 and X2) are bigger, heavier and sturdier than mills like the Sherline. So they are capable of larger jobs. However, they also require a little more tlc when you first get them.
So what should you choose? Well you need to decide what you intend to use the mill for. It is easy to outgrow a mill, if you haven't thought through your planned uses. How big are the things you would like to mill? What materials do you intend to cut?
If you are like most of us, you probably can't afford and don't have a place for a mill for which you would never have a job "too big".