The difference is that one is more formal than the other.

More formal:

David, te presento a Julia = David, I introduce you to Julia

(That's a literal translation, but we wouldn't say it like that in English. In English, we would say "David, may I introduce you to Julia?")

More informal:

David,  ésta / ella es Julia = David, this is Julia

Julia is a woman, so you can't say "esto." "Esto" is only for objects. It is not for human beings. 

To introduce a woman, we can say "ésta es Fulanita" (this is Fulanita) but it's more common to say "ella es Fulanita" (she is Fulanita).

To introduce a man, we can say "éste es Fulanito" (this is Fulanito) but it's more common to say  "él es Fulanito" (he is Fulanito).