If you only plan to hit 25K then stay with UA/MP. You'll earn Premier status as soon as you fly 25K no matter when during the year you cross this mark. With AC/AE you must wait until the following year to get the benefits of status. In Mileage Plus this means earning a 50% bonus of mileage flown on UA, US, CO, LH and even AC. And access to E+ seating on UA flights and maybe an upgrade or two on UA flights with few other MP elites on them. However, if there's any chance of going over 25K then seriously consider AE as you would get Elite/STARGold immediately (if by mid-October) after hitting 35K, much less than the 50K required with MP but most important this includes lounge access even when flying UA, CO or US within the USA, a benefit not included in STARGold benefits offered under the FF programs of the three American carriers (unless you are connecting to an overseas flight).