For the record, before this post I had never heard of Key Lime Air. If it's the one listed on APC under charters then I'd say they're telling you more of the truth than colgan likely is. APC claims they have 31 active pilots and 40 airplanes. I guess with that math all of their "pilots" must be captains and all the FO's pay to play? I obviously never support pay to play operations, however if you aren't one of the guys paying then that would be ok in my eyes. I would keep in mind however that you'll be getting to use that MEI more there than you would at colgan with a properly certified captain.
Colgan Air according to APC has 48 aircraft and 429 pilots. Lets assume that you have to be in the top half of a list to become a pilot (math no one ever bothered to explain to me when I was joining the airline world) So you have to be in the top 215, meaning you won't get an upgrade until 215 guys in front of you have left. You're a CFI now so perhaps you ahven't noticed, but as of lately no one is leaving. When I started at the airlines 3 years ago, I too was going to get a 2 year -ish upgrade. Then the flowthru agreement we had slowed down and stopped, those 75 captains that were supposed to leave never did and I eventually got furloughed.
I DO NOT think you should go somewhere chasing the upgrade, but if you insist, it sure seems more likely that you'd upgrade (maybe even direct hired captain) at Key Lime Air before Colgan. So if that's all you care about there's your answer.

Additional info, I have no idea the work rules at either and thus my comments do not discuss work rules. Either one of those could have work rules that would make it a vastly superior choice, but you'd need pilots from both to find out.