Do the characters in Left 4 Dead have different attributes?

In Left 4 Dead, and also Left 4 Dead 2, does it make any difference which character you pick? Do they have different speed, toughness, accuracy, etc, or are they identical?

In single player, do they have different AI (weapon preferences, more likely to use or share healthpacks, etc)?

Solution 1:

The only difference is in the bot AIs. This Question goes into depth on the specifics of that.

Solution 2:

As I understand it, all the characters have the same attributes except for the model used to represent them. This won't make much difference except in a versus game, and not much even then.