Yeah on/off are best for this kind of thing, because even if it's bad, you can see how much worse it is with someone else instead. And also just individual box score stats like steal rate and rebound rate. Different tools for different jobs.Calculating Individual Offensive and Defensive Ratings | Basketball-Reference.com
How to calculate individual offensive and defensive ratingswww.basketball-reference.com
This is how it's calculated. One of the big weaknesses of the measures (IMO, not much of a stats guy) is that it doesn't always account for bad vs good team. A player on Georgetown is going to have a worse defensive rating just because the entire team was poorly coached and it made even average defenders look bad.
Gonzaga and UConn have similar enough SOS and are well-coached teams, so I think comparing the stats is more likely to have good results.
@auror and others would probably know more than me. I'm making a lot of assumptions here.
If you want to adjust for opponent, something like D-RAPM (essentially an adjusted for others on your team and opponent plus minus) is probably our best bet at the college level.