With UA having a hub at SFO for easier connections, and the most flights to Hawaii of any U.S. carrier, it's hard to overlook them in terms of convenience, but then you'd also be competing with the masses of elites you'd see at hubs, so upgrades could be a challenge, and then you have the side issue of not accruing miles based on distance, but rather on the amount you spend on your ticket (which can range from 5-11 miles per dollar based on your elite level). I'm at the 50K level there and accrue 8 miles per dollar spent (less taxes). I won't fly UA due to changes in their lifetime program, but that's not to say it might be perfectly fine for someone else. Break out a spreadsheet and look at the fares you've bought in the last 12 months and compare AA vs UA in terms of miles you'd bring in.

If you do intend to fly 100K a year, UA systemwides are very different than AA systemwides (you'll buy higher fares to use them and no refund of fare difference if you don't clear).

I'd follow the UA forum for a while before deciding on any changes.