UIltimately they are all just a motor pulling a belt past a few pulleys with a backing plate and a rest.
If the rest and plate are accurately set and don't flap about, the only deciding factors for quality are is it enough power and what belt you use.
My cheap and cheerful belt/disc sander was a great purchase, although it's a bit gutless (OK, it's sold for woodwork, not squaring the end of inch-square steel). Just don't pretend it's a precision tool, but great for deburring and tidying things up cosmetically, or even reshaping HSS tools before putting on a polish.
Neil