i usually have harry say "god" in my fic whereas draco will say "merlin".

Just butting in to say that Draco is actually one of the people who says 'God' in the books (I counted them once because all the 'Merlin's in fanfic were annoying me *g*); AFAIR Draco said it twice, Lupin once and... Fudge(?) once. Someting like that. Asolutely no one said 'Merlin', that's a completely fanon thing.

/butting in :)