I was introduced to "Erza" first (though I pronouce it "Err-zah"), so I go with that.
"Elsa" ("L-sah"?) sounds a bit too delicate and feminine, something that doesn't fit The Titania at all, y'know?
If there's ever a Fairy Tail anime, I'm going to wince everytime someone says her name; Eh-ru-za, eh? Ick, I don't like that at all!