I've used both, although I used Match 7 years ago in a suburban area and I used OK Cupid 3.5 years ago in an urban area, so there are likely to be some differences based on the variables. For instance, when I was using Match, the only other popular dating site at the time was eHarmony, so the concentration of people was probably higher. I should also mention that, although I went on several dates with people from both sites, I ended up in long-term relationships with people I met in-person during those time periods.
I actually found OK Cupid to be the better service of the two. There is a wider variety of people on OK Cupid, in terms of what they're looking for, but I also think the layout helps you get a better sense of the people you're looking at. I was pretty accurately able to gauge the seriousness of women based on the way they answered questions, the number of questions they answered, the things they wrote in their profiles, etc. Match, at least when I used it, was based mostly on what people wrote about themselves, unprompted, and that didn't necessarily tell the whole story.
I also felt like there was a greater variety in terms of background and interests on OK Cupid. Again, this could have had a lot to do with location, but it seemed like the majority of the women on Match were either teachers or nurses. And that makes a lot of sense—those are two professions where people tend to be on settled career paths, are therefore more stable and less likely to look for flings, have very little possibility of meeting people through their jobs, and work irregular hours that aren't conducive to meeting people through evening activities.
On Match, I had a lot of good email conversations with women, but very few of them turned into actual dates. That was partially, if not mostly, on me, but also, I think, because people seemed to have less time overall. I ended up dating one girl for a few weeks before she acknowledged she wasn't feeling it (which was fine because I'd started to get involved with my now ex-girlfriend at that point). On OK Cupid, I'd message a few times and then we'd end up going out. Some of the dates were total duds, others led to strings of dates.
Overall, the people on Match probably were more serious, on average, but that made them more selective about going on actual dates. Personally, I think that OK Cupid was the better of the two, but your mileage may vary.