I would mainly fly domestically (if at all internationally). Like someone above mentioned, most of them would be NY, LAS, or MCO-type flights for family vacations. That being said, my miles would, most definitely, be used for status with *A or US Air. So I guess the question lies between US AIR's treatment of me or easy lounge access with the foreign program referred to above (except having status with them would most likely be useless, as I won't be flying on their planes at all)
To be honest, if you were flying from an airport with Southwest (WN) service... I'd be recommending you fly and accumulate on WN. Seriously. They arguably have the best ability for you to do what you seem to want to do (travel domestically, occasionally get an free flight somewhere). Is flying Airtran a possiblity until Southwest takes them over?
You need to fly 25,000 miles a year to maintain base-level status in most programs. That can be done on leisure travel... but it's a lot of leisure travel (5 round-trip transcons in a year or so, 10 round-trips of 1250 miles to and from a destination).
Your chances of upgrades from US coming out of CLT are probably not great at a base elite level. The main benefits are going to be things like early boarding, free checked luggage, and bonuses on flown miles- many of which would also apply if you were *A Silver/Gold in some other program, but I think
sbm12's advice (mostly) holds true. I think the biggest problem for you is that US at this point doesn't allow partner redemptions online, and those are the main way you're going to have to redeem for domestic trips (US's availability is really terrible). That means dealing with US customer service over the phone... really suboptimal in my experience, plus
their fees on award tickets are really horrible (which is one of the reasons why if someone wants to use their awards domestically, I recommend WN: NONE of those fees applies to redeem a Rapid Reward).