For which player is selected as the "ISO weapon" as a heuristic, think about the best sum of IS and DR. (I'm pretty sure it's not flat IS, because when playing IS my teams have tended to favor my PFs over my C's).
For how I choose between them:
1. Good IS offense is more reliant on quality offensive flow -- e.g. the ability of your guards to pass the all inside and get assisted shots.
2. Inside Iso will generally result in a slightly better 3FG%. If I think I have a potential guard mismatch I'll consider it.
3. If I think I am a slight underdog and the game is gonna be tight (like it's gonna go down to the last minute), I've become a bit reluctant to use LI, because players will take worse threes when trying to catch up.