The NBMEs are supposed to be the gold standard for figuring out where you stand, though some people may tell you that their UWorld Self Assessments (UWSA) were pretty predictive as well. I'd heard/read mixed opinions on the UWSAs but opted to purchase them and was pretty happy that I did so. If you buy NBMEs with extended feedback, you can view your incorrect questions but aren't given the corresponding right answer or an explanation, so you have to figure it out yourself and/or do a bit of digging online to find a site or thread where other people have discussed/posted some of these questions; depending on how much you have to review, this can be a bit time-consuming. The UWSAs spare you this chore since you get the answers and explanations like any other UWorld question block. So at the very least, you're getting 8 extra blocks of questions if you have both UWSAs (and a provisional/estimated 3-digit score at the end, of course).
The curve on the UWSAs is also different- you may get a lower percentage of questions correct in comparison to an NBME but may wind up with a comparable or higher score.