I have a delta skymiles AMEX, but the $95 annual fee, bag fees, and more expensive flights don't always make it worth it (although as a cardmember you do get 1 free checked bag). We fly Southwest almost exclusively, and my DH has the Southwest visa, so we try to use its perks whenever we can. IMO, not having assigned seats is a small price to pay for better schedules, flight prices, being able to check 2 bags, friendly crews, and that all being said, I've never had to sit away from my family (even though there's only 3 of us).
If you're concerned about not sitting together, spring for the early bird check-in for $10.