Definitely do this. The 757 seat is pretty decent for a daylight flight and is good in the relax/recline mode, it's just less than ideal compared with a flat-bed for sleeping.

With that said, it's definitely worth having a pair of decent seats across both flights as opposed to having a good seat on the TATL sector and a terrible seat on the transcon (which really isn't much shorter). In addition, service on the A321T aircraft out of JFK is considerably better than the standard domestic product that will be offered on US from PHL. Finally, the lounges in JFK are a considerable improvement over what is on offer in PHL.

The non-transcon A321s (US and AA) are both rather uncomfortable in my experience. Legroom is pretty bad, but especially on the US aircraft, recline is non-existent, and seat comfort leaves a lot to be desired. The full-flats on the A321T are far far preferable.