I don't get it what the problem is. If you rented the game at big discount on Steam then just wait for a big discount on gog.com (75% off is usually the best you can get here) and then buy it. That's what I do. I have a moderate sized Steam backlog from before which I'm slowly converting to gog now that more games are here and sales are best option for that.

I dunno, for me it depends on the game, what extras it has, and whether or not I'll get any benefit from a DRM-free version (for instance some games I like to store on a portable hardrive so I can take them them places :P). For games I know I want to own and replay, I usually buy on GOG, either as a first copy or a "collector's" version. Or if there happen to be some nice extras (especially wallpapers).