As someone who tends to shuffle up and down between low-tier and mid-tier on UA and AA, I can say that life gets a lot nicer at the mid-tier level on either airline.

Standby is the biggest perk with today's load levels. Upgrading at 72 hours is also very worthwhile: since there are a lot of mid-tiers in this world, they really do soak up a lot of the upgradable inventory in that short period before the low-tiers get their shot.

If you receive more op-UG's as a low-tier than a mid-tier, I'm fairly certain that's due to random chance. My experience has been better as a mid-tier. From reading accounts of how GA's do op-UG's, status definitely plays a role.

And of course 37,500 extra bonus miles is kinda nice too.

United has the added perk of getting more experienced, U.S.-based phone support. I'm not sure about AA.