JDiver

Which FFP IMO should depend on where you'll be and therefore which airline will serve you best. E.g. SEA is great for Alaska, BDL or TPA perhaps not as much. Look at it in the long view.

Though DL has the "SkyPesos" reputation, I think award availability has changed among the "big three" to the extent it really doesn't vary too much in terms of availability. It's more about partners and diligence now.

Earning status is based on minimum spend on all big three now, so there's no major advantage (though DL and UA both waive some ticket spend for credit card spend, and AA doesn't yet - speculation is this has stalled as AA negotiates with both Citi and Barclaycard).

As to flying, many would say DL is most consistent in service, AA has deteriorated with being taken over by HP dba US and UA seems to continue as it has been.

If you're not doing Star, Alaska does allow miles earning on both AA and DL.

And for domestic short and medium haul with a very consistent product and good customer support, there's always WN / Southwest.