Without some useful personal benefit, such as time to shop, meet friends, engage in cultural activities, gain tier points, or save money it is clearly better to go direct and avoid the delay, hassle, and risk of passing US immigration and customs. Given an even choice between direct or via USA, avoid the USA, every time.

If you've got the spare time, would like the tier points, would enjoy going shopping in New York and you personally face little risk of immigration issues entering the USA, then go via JFK. It sounds like all these things are true for you.

As for seating, if you need to sleep on the return flight then don't fly on an AA operated aircraft. Between the dinner service with the cabin lights up, the breakfast service with the cabin lights up, and the always-on galley lights shining through the translucent galley curtains (US regulations forbid US carriers from having solid galley curtains), actual sleep time on an East Coast to UK flight operated by AA is only 3 hours of bad sleep. No amount of all aisle access makes up for that if you want rest.

Frankly, since I like my sleep, even if I wanted to shop in NYC and maximise tier points, I'd go via JFK outbound and direct on the return.