As a romatic, i'd love to say that i believe in instant romantic chemistry. But, from my experience, i can say that it's not always instant. Sometimes, as it happened to me, you can know a person for years, you can be just friends with this person and never think of her/him more than a friend. Until one day you look at her/him and become more that attracted to her/him.
I think instant romantic chemistry lives longer, since it's appeared out of nowhere it has no particular reasons to go away, while the other one appeared because of particular moment/thing/feeling and may disappear without even noticing you.
In the end, chemistry IS chemistry. You can't hide from it, and maybe there is no difference whether it appears instantly or after some time.