I'll ask the same thing I always do... "What's your goal?" I assume flights... but where would you like to travel (and in which class)? It appears that you probably already have some UA miles and/or points at Chase... so the Sapphire might fit nicely into adding to those piles of points. All of the above are solid bonuses... do you spend much on credit cards?
I like the flexibility of the Sapphire Pref., but I also fly on United...so the new Chase UA card with a couple of lounge passes, free bags, and priority access is nice (as is primary rental car insurance). I consider these benefits to be much nicer than the AA cards, which are basically just a big bonus up front and not much else.
My personal take is that I wouldn't go wildly crazy on churns given your relatively brief credit history. (Not saying I wouldn't do a couple per year... but don't apply for 6 cards in the next year).