I'm currently loyal to United and have Silver but plan on getting a lot more miles in next year furthermore I am also moving to the Bay Area (SFO/OAK/SJC). I've heard that SFO is notoriously hard to get upgrades out of based on status. My question if I were looking to maximize my upgrade potential which airline should I focus placing my miles on?

Thanks

How many is "a lot more"?

The 25K and 50K levels don't give you unlimited upgrades on AA. You do receive an allotment of upgrade instruments you can apply to flights you want an upgrade on, and you're more likely to clear on those than a Silver on UA would be. But you won't receive enough instruments to upgrade every flight you fly, so you need to be selective about which ones you want to apply them on.

Anecdotally and from my conversations with friends and just reading FT, DL customers seem to report getting upgraded a good bit. I don't know too many US flyers, but they seem to be relatively happy with the crappy IT systems that let them snag upgrades they probably don't deserve, but then they sit up front and don't get fed on medium-length flights, so it's not much of an upgrade. But those are mostly reports from top-tier flyers; I'm less familiar with lower-tier flyers. AA's top tier has mixed reports; many dissatisfied UA flyers jumped over to AA and initially reported great success with upgrades, but then I started hearing some of the "grass isn't always greener on the other side of the fence" reports, so I don't know.

The one thing to note is that if you move to the Bay Area and choose someone other than UA, you're instantly limiting yourself to the number of flight options you can choose, as instead of hundreds of flights each day non-stop to the destinations you want to go to, you're limited to a dozen or whatever flights a day just to the other carrier's hubs, necessitating a connection to go onward to your final destination. If you don't mind the connections and more limited flight times, you can have a good experience being a non-hub-captive elite (better upgrade rates, less crowded clubs, etc.).

"Which airline will give me the most upgrades?" is a good question to ask but it's not the only one that's important. You should also consider which airline or alliance offers the best routes. That depends on your travel pattern, and the answer could be UA/Star Alliance, Southwest, Virgin America, or something else.