This year this is what we did - previously doing the BOS->SJU->EIS. I waited too long to book flights and ended up having to go thru STT and go a day early and stay a day late. Stayed in STT those nights and it was easy peasy. And we never found the return thru STT to be onerous. We flew back on a Thursday and it was not busy at all (though that Thursday thing might have been part of it)