Welcome to FT!

I'd go with the American (which I believe is a 737-800). The seat is roomier in every way and probably has a bigger tray as well. And if it's one of American's newer domestic first class seats (which is common but not universal throughout the American fleet) it has the nice feature of sliding forward even as it reclines, so that the person in front of you doesn't recline back into you much.

The one superior feature about the United option for this route is that on the left side of the plane the seats are stand-alone, so you're not sitting next to anybody. Still, as much as I personally like that feature, I'd go with American for this route.

I'd also suggest a non-bulkhead seat on American as you can then store a small bag in front of you and in effect have more legroom than bulkhead. But opinions vary on this.