I don't know enough detail about BPI (or others) vs. RPI for assessment accuracy, but most of what I read from anyone in-the-know, it's apparently a better system. Maybe it'll be used more (or exclusively) at some point, but RPI is the computer tool for now.
What I do trying to build a tournament field (or "guess," I guess you could say), the further into the season it goes, I adhere less strictly to each team's pure RPI, and also take the RPI of tourney contender opponents with a grain of salt. By mid-February, some of it doesn't hold up with a pure top 25/50/100 etc., and I try to consider how true to the ranking each win/loss is, relative to a team's potential inclusion in the tournament (and relative to pecking order).
Random example: Belmont's RPI is still 23. The sole top-50 RPI win is vs. Middle Tennessee. Two top-50 losses (KU, VCU) are fine.
Now, Belmont is out as an at-large regardless of decent numbers, but this applies more to when that team shows up on other teams' resumes as a top-25 win. It doesn't remove it from consideration as "quality", it could have much less weight than a win over a 40 rpi team that has several top-25 wins vs. win over a top-25 with a low-impact resume.