If I had to chose between the two - and having quite a few good friends, definitely that. If you just have an SO your in a really bad position, you need them too much, and if they leave your high and dry. Good friends will also get you out on the weekend, sometimes the weekdays, if your dying to go out and do something you've got people to hit up, an SO may be too tired from work, may not have all the things in common or want to do the same things you would, with friends you can call around and even if a few aren't down for it you'll always have one or two who'll be like "Yeah, that sounds like a plan. Just let me handle a few things around here and I'll holler at ya when I'm done" (and no, I'm not insinuating a blow-off).
The other thing, just in terms of your own social skills and staying current with what's going on, the friends will be better than that and being of the same gender quite often (well, if your gay or lesbian this may not matter) it means that you don't have to worry as much about maintaining gender roll just because - you get more than enough assistance in peer pressure to do guy things if your a guy, girl things if your a girl, and if you were a little too on center with that to where it wasn't generally a good thing; there ya go.