That's where UA might work against you as there have been posts by some in the UA forum that they meet the flight requirements (say 50K at midtier) but not the spending requirement ($6,000 at that tier). If you're buying discounted tickets weeks out, it could be a hard target to meet. AA, on the other hand, does not have spending minimums attached to their tiers right now. Not to say they might not implement that down the road, but not as of today.