Someone posted in another thread that Tony didn't seem very fast. I think they said he didn't seem Portis fast.
IMHO- These guys have two very different running styles. Portis is a guy that is fast regardless of the direction he is moving and once he gets going up field, he is quick to his top speed.
I watched all four of Tony's college games and he reminds me of Robert Smith of the Vikings. More of a hundred meter guy that isn't going to get through the traffic as fast as Portis but if he makes the corner, he just kind of sits back and eats up the yards with long smooth strides.
I predict that if Davis has another good year and Tony is only getting 10-15 plays a game, the Texans may try to incorporate him in the passing game more to take advantage of his speed in the open field.