The RC and OD&D are both very good and you can't really go wrong with either of them. I'd recommend getting both, reading both, and deciding which is a better fit with the sort of campaign you want to run. The RC has a lot more rules, which can be a good thing or a bad thing, depending on your perspective. If you want a campaign that goes up to level 36 (as opposed to plateauing around level 10-12) then the RC is definitely the way to go. If you want simple/straightforward dungeon- and wilderness-crawling then either will work, though OD&D probably has the edge (both because you'd be ignoring large chunks of the RC, and because (IMO at least) OD&D has better instructions and advice for running this sort of game).