I think you may get something like 4 extra Avios with the AA flight, BA takes a short cut.
I think you may have got the flights slightly mixed up, BA283 (IB4667) leaves around midday and is a jumbo at the moment, whereas AA137 (IB4238) leaves at 14 hrs. The 10:25 is with (gulp) United. The BA flight leaves from T5, and AA from T3. The earlier you get into TBIT the faster it is to get out, unless arriving very late indeed, as a very general rule. It's probably worth paying a bit extra for that, unless you have a US passport or Global Entry.
Having said that, it's always good to double-check the prices on AA.com - sometimes it throws up cheaper options, albeit with different conditions.