Just some things I've noticed:
Marlton builds better quality stuff -- turbines seem to be almost definite
Samuel seems to get more powerups -- I could be just getting lucky but it SEEMS as I can count on powerups almost being psychic with him in earlier rounds
Russman -- Not sure, but in groups of 4, to me it SEEMS like I get less zombies spawned near me...could just be coincidence but I don't know
Misty -- This I'm not sure of..I THOUGHT perhaps her knife quotes indicated she's more skilled with the knife...I love my knucks so more testing is needed, Bowie Knife doesn't seem more powered up but I remember getting one hit kills with Ballistic Knife up to round 6 or so..seemed strange.
Finally, I've been shooting Denizens quite a bit before they hop on my head....after about 10 in a row, to me it SEEMS as if they are jumping on my head HALF as much...notice it especially from the Bus Depot to the Tunnel, and then to the Diner..I get jumped on no more than once....
I dunno guys...not trying to stir up controversy, just keep your eyes open and see if any of this rings true for you.