seats on 747 are a bit tighter in standard coach class (31" pitch and 17 wide) vs. 787 (32" pitch and 17.3 wide). Also, no personal IFE on 747 v. personal seatback entertainment on 787 which could be significant if you haven't prepared or aren't carrying your own.
not UA specific, but someone else may want to advise on clearing CBP at DEN v. SFO as that may have some weight as well.