By May or June, the 52 'regular' (i.e., non-ex-Gulf Air) DL 763ERs (and JFK almost never gets an ex-Gulf 763 on a scheduled basis - only as an equipment sub) will definitely have been refurbished (i.e., the 'newer' BE seats with AVOD PTVs, etc.) - there are only 2-3 of them still with the 'old-style' seats today anyway.

If IFE/PTVs matter to you, then DL may be a better choice since CO's refurbishment of their 757s with AVOD is still ongoing (so you have a chance of getting the old looped-programming system - according to the CO board, only 13 of 41 have been redone).