I would not pay more for WT+ in these cases, unless you had a stash of BA Avios to use to upgrade from WT+ to CW (which is possible from any WT+ fare). WT+ will provide slightly upgraded food and a modest amenity kit, with seats that are more or less on par with, say, the port side first class seats on an MD80 (with a slightly narrower partition between seats). 31A/B/H/J have much more legroom, and row 11 on the 763 is also quite roomy. Given how short these flights are (in my opinion) I'd stick with AA overwater.

If you end up buying them as BA codeshares you should still be able to get premium seats to which you're entitled, though I must admit that the last time I did such a thing was before MCE was out so I was just booking exit rows and had no problem--perhaps someone with experience getting MCE on a BA-marketed AA-operated flight can weigh in here?