How much will your airline ticket purchases be driven by price vs. loyalty to one airline. Will you be loyal to UA or AA if they cost $100 more per trip.

Since you plan on doing only 4 trips per year, you will be just under the first tier elite level. Creative routings might help.

UA probably has the most nonstops and has economy plus. If price is important, you might want to choose AS. AS has a lot of partners and will allow you to accumulate miles into one program while shopping for the price.