Honestly, it really depends on the routes you fly, unless all your travel is bankrolled by someone else.

I flew out of CMH for two years and was NW elite both years. It worked for me because I mostly did west-coast RTs for work and the DTW hub was the most convenient option.

My take was that CMH is a lucky place because its a short hop to a hub of most US majors (DTW, CVG, ORD, EWR/CLE), and plus DL has significant operations based in CMH itself. I've heard that in the days of the AmericaWest hub, it was even more competitive.

So anyway, it comes back to what your routes are, and what you really value.

Free upgrades ? CO or NW, DL maybe (no personal experience)
Better (?!) FF programs ? AA or UA

Edited to add:
If CO is treating you well and you're not constantly stuck on those regional jets, there's really nothing wrong with them!