I wouldn't call 1991 Dave Brown a "prime Dave Brown" - but it was definitely a prime Probert.
Brown's career record according to dropyourgloves.com is 107-16-36 (.786). Only six of those losses came before the end of the 1989-90 season. It was after that, that the cracks started to show: He was 30-10-16 (.679) after that, meaning he was 77-6-20 (.845) prior. That's his prime.
these W/L records are subjective, but let's assume they are useful. Probert faced the same tough competition as Brown and posted a career record of 154-51-62 (.693) - In his prime years before cracks started to show, he was 74-14-22 (.773) and 80-37-40 thereafter (.637)
So the truth is, we never got to see a REAL fight between these two, in the 5 years that they were both in their prime. Even if we did, one fight wouldn't tell us much. I'd prefer the much larger sample size of how much they pummelled the league's other heavyweights. Brown scores a clear win in that category, both prime, post-prime, and career.