Dear MaryAO:
Forget about DLCrap!!!!!
Their FF program has been totally decimated by some of those execs earning huge bonuses so they "do not pursue other interests."
DL will have your miles expire if you do not fly them in a 3 year period.
I suggest you use your CO OnePass. You will be able to earn miles if you have to fly that hideous DL, plus, you can earn and redeem on CO, NW, KL and AF (on some routes through IAH). CO miles NEVER expire and they will count actual mileage flown towards elite status on all types of fares (not 1/2 like DLCrap on discount fares).
PLUS, CO goes into A LOT of places in Mexico (more than US, DL or AA). Another factor is if you fly DL into Mexico, you run the chance of being stuck in a 3-hour flight in one of those wretched CRJ's. I am sure you have flown in one of them out of MKE.
Concentrate on committing your flying to 1 or 2. You may went to consider using CO and AA. AA is very good for the Caribbean. The only minus with AA to the Caribbean is connecting in MIA. SJU is much better to connect to the islands.
Good lukck!!!
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Do not sit in the back.