It will depend on your flying pattern.
For example, if you fly mostly short-haul, about 30 segments a year, you will be DL Silver (FO) or FB Gold, which grants you lounge access. Because, if you fly short haul (500-600 miles per flight), you will qualify through segments.
If you fly long haul, DL SM should be better as you will always earn 100% of the mileage when flying a paid ticket on AFKL.
About the mileage earnings, yes, you will earn less with AFKL than with DL, but FB allows one-way awards, and you have the promo @wards, a 50% discount on the required mileage for an award.
About the mileage expiration, as long as you take 1 flight every 20 months, the validity of ALL your miles is extended of 20 months! Except if you stop flying for a long time, you should not bother.