Do a typical trade for equal amounts using both methods at the same time and see how much EOS you end up with, that's what I do sometimes when I want to know quickly if one method is cheaper. Also the Bittrex withdrawal fee is a flat fee whereas Blocktrades is (or was at least when I calculated it) based on a percentage, so Bittrex would definitely be cheaper over larger amounts.
But your way has the benefit of being through your own wallet so there's also that to consider.
Do a typical trade for equal amounts using both methods at the same time and see how much EOS you end up with, that's what I do sometimes when I want to know quickly if one method is cheaper. Also the Bittrex withdrawal fee is a flat fee whereas Blocktrades is (or was at least when I calculated it) based on a percentage, so Bittrex would definitely be cheaper over larger amounts.
But your way has the benefit of being through your own wallet so there's also that to consider.