For the team in question: I would guess beating a top 150 opponent, but am not 100% sure. I'm pretty sure there's no "quality loss" section of the PR, but I'm not sure if the PR cares much about RPI 100+ wins.
For that team's conference: definitely, definitely beating the RPI 150+ team. Your conference mates' RPIs will be determined 50% by their opponents' records (including yours) and only 25% by their opponents' opponents records. But while that 50% is dispersed 26 ways, that 25% is dispersed 26 times 26 ways. Which means, unless I'm having a brain fart right now, a one-game swing for you is equally important to your conference mates as a 52-game swing for your opponents (at least, for the opponents that are not also on the schedule of your conference mates). Assuming the 150+ RPI school does not have a record that is 52 games better than the top 50 opponent, your conference is better off when you beat the top 150 opponent.