As long as you own both combinations, it's just a matter of which one to standardize into your branding scheme, which is a MUCH better position to be in than only owning one or the other.
If Rule #2 of domains is "In Most All Cases, Hyphens and Numbers Suck" (with rule #1 being .com, .com, .com), I think this is one of the extremely narrow cases that necessitates the "in most all cases" caveat, mainly because 1) co-op (hyphenated) is a standard way to communicate that particular abbreviation and b) absent the hyphen, co-op spells "coop" which could conceivably bear negative relevance to whatever condo/apartment/house related suffix that might follow.
Like, "coopapartment.com". Who the hell wants to live in a 'cooped apartment!'
Maybe during beta, you could vacillate between branding strategies and see which one has a better response (obviously, making sure to have both "co-op" and "coop" in your main text body and tags). The general level of sophistication amongst your target users (co-op buyers?) is going to be higher than average Joe, so I wouldn't get too set in stone with one or the other since they're going to understand that "coop" in this context doesn't relate to "cooped". With the hyphen, you should get a moderate SEO/url advantage in that you hit square with both co-op and co(space)op.
G yeilds "co-op" at 19.4m, "co op" at 19.6m with "coop" at 30.7m.
When you run "coop" -chicken , you shed a couple million results...
[/stream of consciousness drivel]