I fly JFK-SFO/LAX quite a bit on both UA/AA.
As far as coach product goes, coach seats are coach seats. United is nominally all E+ in the back, but it's not a full-fledged E+ product (it has the lowest pitch across the entire E+ fleet). As an AA Plat, you'd be able to get an exit row on the 767's AA is flying.
As far as business class goes, the seat is slightly wider on United (
JFK-SFO on UA)than AA 762 (
My review for JFK-SFO on an AA 762). Otherwise, my preference is for AA (in terms of service, food, friendliness of staff, etc.). UA works the business class cabin with 2 flight attendants; AA works their cabin with a small army. The AA 763 flies with their now-old next-generation business class product (an angled lie-flat; my commentary with respect to the product for
JFK-LHR).