Sonic will likely sell better. It has the whole exclusivity deal going for it.
Watch Dogs delay and the fact it was released on other consoles beforehand will easily eat at the potential sales it could have had. This and the general mediocre consensus for the game.
Sonic does not have good reviews either mind you, however it is exclusive, and the Sonic die hards are likely to pick it up. Plus uninformed parents will likely pick it up for their children for christmas to go alongside the Mario Kart bundle or something to that effect.