You predicted a 3.00 ERA for Zambrano, and I think that's probably about right.
From '01-'03, Hudson's ERA averaged about 3 as well. (It may be worth noting that his ERA went down each year, from 3.37 to 2.98 to 2.70.) In 2004, he was injured, which probably accounts for his higher ERA (3.53).
But now that Hudson's in the NL, his ERA could drop to around 2.60 or so. That's a fairly significant difference. (Of course, if Zambrano's is 2.75 again, then it's not such a significant difference.) I would guess his WHIP would be a bit lower, maybe about 1.18 or so.
Based on what I'd project for each pitcher, I'd take Zambrano by a hair, since his K's will more than make up for the difference in ERA and WHIP. But I also feel like Hudson's just a bit safer, so I'd probably lean towards him. It totally depends on the situation, though - if my team needed strikeouts, then I'd obviously want Zambrano.