Neither team played their best basketball at the most important time of the season, which is why they're in this situation. The case for Brandon (28-8, 4th in RPI) is that they played a tougher schedule. They beat Calgary twice, but were otherwise 1-4 against tournament teams (although that includes a 14-point win over UBC and a one-point loss to Carleton).
The argument for Acadia would take in the fact they beat the Bobcats in their lone meeting at the Wesmen Classic in Winnipeg over the Christmas holidays and are 25-6 on the season. Three of those losses, though, came against the same team, Saint Mary's, and Acadia is just 9th in the RPI. (As a footnote to that, the Atlantic University Sport powers-that-be might want to review their rules regarding pre-season practice and games ... there was some carping about that on the Dale Stevens discussion list back in the fall, and one wonders if there's a correlation with the RPI of Atlantic teams. Other than Saint Mary's and that one holiday tournament, Acadia hasn't seen any of other Final 8 teams.)
Meantime, cishoops.ca has the lowdown on Brock clinching the final OUA spot beating Ottawa.
Something else worthy of discussion is whether it's time to look at a more compact playoff schedule. There were a couple comments (from Chris Kallan and Mark Wacyk) that teams looked sluggish and that their game lacked finish.
Acadia had almost two weeks off before the AUS Final 6. Now their destiny is out of their hands. In the OUA, Carleton, which will have to play three games in three days to win the Final 8, has only played three times in three weeks. (Then again, in the days when the OUA held a Final Four, that would have meant giving up home court for the division final, which is kind of silly.) The Canada West reps at nationals will also go 12 days between games.
The players are students and there are logistical things to work out, but is having that much time between the conference playoffs and the nationals really best for the game, especially in a sport where so much depends on being in a good rhythm, having that fine touch?