CP: My experiences on CO:

1. Ease of upgrades. Domestically: if you fly full fares, if you're gold (middle tier), you can confirm an upgrade with no coupons, certificates or stickers. Silvers I believe can upgrade two days before the flight. For less than full fares, you can upgrade by calling a day or two in advance depending on your level. You also have the same award privileges on NW and HP. (I think when they are finally done with the remodel at DTW in 2001, NW will get a lot better.) International: dicier, some fares cannot be upgraded even with miles. I believe certain domestic classes on DL cannot be upgraded at all.

2. International comfort and service. Cabins are two classes only (same as DL) and the service is excellent.

3. Airlines convenient to Tampa. Just a comment, there was a discussion recently on the problems at MIA. Most people seemed not to like MIA very much; you might want to find the discussion and consider if you really want to be flying through there a lot if you can avoid it. EWR has nice facilities but a bad on-time record; I think it was rated worse in the country the last survey I read.

4. Overall positive experiences with the airline, also recognition as an elite. For my money, CO crews are the friendliest. I nearly always feel like they try hard to give the pax great service. A huge plus for me, since I nearly always check baggage is CO's superior baggage service. They give elites special tags and at most of the airports I fly into, my bags are nearly always among the first onto the luggage carrousel. This one perk has saved me hours waiting at baggage claim the last two years.

Sounds like a tough choice between DL and CO. As a gold medallion a couple of years ago, I always had trouble getting domestic upgrades on DL, and I sometimes had to pay for them by buying points. I am almost always upgraded on CO, but I've always been top tier (gold or platinum, which was introduced only this year). OTOH I'd rather fly through ATL than EWR, and DL's international partners are a little nicer than CO's for the most part. I hear great things about DL's BusinessElite service, sounds at least as good as CO's BusinessFirst.

Another thing you might want to consider is whether TPA has a Crown Room or a President's Club. Either is a big asset when one flies so much.

Good luck with your decision and let us know which way you decide to go.