I'm 6'4" and would always pick row 31 over row 20. Having said that row 20 is a better than average bulkhead row as there is a gap at foot level. Personally I'd take the row 31 (no) window seat rather than the aisle as it is a high traffic area.

As a wild card though, what about row 32 ? It is one of the only standard rows in MCE in the 77W, and I find the leg room in a standard MCE row adequate. If you can get the pair of seats to yourself it might be a good bet.