For July '08 - Jan '09 chances are VERY high that you'll continue to collect miles in SkyTeam regardless of which carrier you book on. July is a sure thing and January is only slightly less.
CO generally has one of their AVOD planes on that route, though it is not guaranteed. I'm pretty sure that DL has switched to the 77L for this route so you are guaranteed AVOD. The pitch and everything else is about the same. If you want MQMs you'll risk only 50% credit flying on a cheap fare on CO, so that may make the DL option more compelling.
I'd shop based on price for your desired dates and just buy based on that.