Single-player campaign because it can have more content in it. A cooperative map has to fit on one map, while a campaign can have as many maps as you can fit in a file. Sure, loading screens are a bit annoying, but you have much more to work with. More complicated trigger systems available. Just more to do. I'd love a cooperative version of the Rexxar campaign, with each person controlling a different character, but that's just not possible in the normal engine because of how campaigns are structured.
If you can layer all the maps of a Rexxar-style campaign into one map file and shift them to make the full campaign's worth of content, then I'll go for the cooperative. But potential content is king, and thus the single-player campaign is the way to go.