The UA seat is 3 inches wider but also has up to 6 inches less legroom. This can be important if you're tall or wide.

Another thing to consider is that the ERJ-170s don't have ovens so no hot food on the UA flight. AA will have ovens on their 738s and on a 4 hour flight most people like a snack or meal.

AA will have the more traditional domestic first class service but the UA flight will be just fine. I do enjoy the 1 lone seat on the left side of the UA ERJ-170s when traveling alone and find the service to be fine. United will have Wifi on their ERJ-170s if that is important to you. UA has an edge of afternoon frequency travel which may fit your travel plans well.