Character-wise tekken's characters have always been pretty much rip-offs of characters from other series, including street fighter.
I don't think so. Street Fighter itself is the archetypal rip-off -- see "Master of The Flying Guillotine" and countless other *much older* martial arts movies for reference. Street Fighter just started the boom. Tekken's bigger problem is ripping itself off; there are still a lot of shared moves/techniques between characters. It's much better than the past days of model swaps (Yoshimitsu/Kunimitsu anyone?) but it's still there.
... and tekken is a grappler, more closely related to wrasslin games.
Huh? In that they both... have throws... ?

I'm not following you here. Please elaborate.
Anyway, I don't think the engines are all that drastically different. I mean, you could get nitpicky and start talking about super-cancels and "chickens" and other pro-level "timing" ---steaming pile of meadow muffin---... what it really boils down to is how they decide to handle jumping and projectiles (both of which don't feature too prominently in Tekken) and movement on the z-axis (which isn't in Street Fighter.)
I like both games. I like both stories. I'm interested in seeing how they (Cap and Nam) each decide to integrate the worlds, particularly regarding the above mentioned gameplay idiosyncrasies. We've got a damn long time to wait though, so I'm not getting too excited...