They are different species of the same family.

Passiflora plants and fruit have their own forum here. Link below is to a discussion of exactly this question.

Dave

Here is a link that might be useful: Maypop's and Passion fruit

P.Incarnata has nearly taken over parts of my garden. If I had known how invasive it would be I wouldn't have planted it. The flowers are awesome, though.

Just wait, the gulf frit butterflies will lay eggs on them and the caterpillars will eat all of the vines. I would also think in your zone they would die over the winter and not come back.

Not at all. Maybe a cold enough winter will come along (which would be ok by me) to eliminate them, but maybe not. This is their third growing season now, or maybe fourth, I've lost track.

They are reliably hardy for me in Michigan as long as I throw a good mulch layer over them. They bloom well, but I don't ever get ripe fruit in my shorter season with cool late summer nights. Too bad, I hear they make a great addition to things- even the semi-mature fruits have the characteristic passion fruit aroma when cut open.