Thanks for all the good feedback.

The plot does thicken.
My company has a contract with United for TATL.
And another with CO for ANC and YYC.

They are pushing UA, but I agree with Monitor, UA really sucks for South Florida. Looks like I need to depart from MIA (sometimes FLL) as United insists PBI connect through IAD or CLT.

So my thoughts are Delta is a gonner - My company has sold it's soul to *A.
Now it comes down to CO, USAir or UA program. I'm leaning towards USAir as I can get shorter duration trips to LHR and leave directly from my home airport.