I'm doing umat this year too so I don't have much experience, but if you can afford it, go ahead and buy the Acer ones. Acer is the company that comes out with the official umat papers every year so their practice papers would be the real deal, although this is to just get a better idea of what will be on the actual paper, since you can't really prepare for umat.
Other companies try to emulate this, although I'm not sure how successful they are (UMAT prep course industry has a monopoly on all of us naive students). From reading through atarnotes, some people didn't do well in their umat even though they'd been getting high percentiles in courses such as Medentry. Conversely, others were able to achieve a higher percentile in umat after practicing with such prep courses.
If you just want to get a feel of what the actual papers are like, I think that after you register for umat, Acer gives you a free copy of one of their practice papers.