If Igawa gets 7.89 runs per game of support, he will, without a freaking doubt, win 17 games. (Unless he's hurt, and this assumes that he will replicate Johnson ERA of 5.00, which I think he will come pretty close to matching)
For him not to win 17 games, he'd have to pitch so badly in numerous games, that his ERA would probably be in the Kyle Snyder range.
Wins are a pretty poor measurement of a pitcher. I'd take the Unit because of his low WHIP, and high strikeout numbers. I think he's the better bet for 2007. If Johnson came back to New York, he could possibly win 20 games.