It probably makes the most sense to join the FF of the carrier that you will fly most often in the future. CX Silver needs 30k mi but will get you lounge access when flying CX.

AA Gold will let you earn upgrades on AA metal and makes picking exit row a bit easier. As Gold the upgrades hard to earn and hard to burn, though.

Both would give you OW Ruby status, which allows use of J check in and a few other perks, though not much.

The 4 sector requirement on AA has anecdotally not always been enforced, but it is official.