As a tall person, I don't think there really is any significant difference. The seat pitch in economy on most jetblue flights is 32" (per seatguru), which is generally the same as united/aa on transcontinental. Yes there are UA/AA/etc layouts with 31, as there are jetblue with 34". The bottom line, is that if you don't fit in these smaller seats, paying the upgrade fee on a crappy airline/ticket generally gets one more space than flying a more premium airline. But the difference is frequently nullified (cross airline) simply by selecting a different date/flight with an alternative plane/seat layout.

(and the widths all tend to be ~17.5" or so)