My wife and I got a Vibe for her to drive when we found out we were going to have a baby (my son, who will be 1 tomorrow!) and I have no complaints about the car. We've had it for 16 months now and don't have any squeaks and rattles, though I know others have reported them. Maybe we're just lucky. It's slow, but I like the handling and with my wife (lightfooted) driving it about 90% of the time it gets 26 MPG in the city. I will say this, I haven't found anything child related yet that won't fit in the thing. Got one of those "Little Tykes" turtle sandboxes for a my kid for his birthday and fit it in the hatch without putting the rear seats down. It's also very easy to get a child seat in and out of.
My parents have a Malibu Maxx with the 3.5 V6 and it's a very nice car. Like Capriceman said, handling isn't sporty, but is fairly refined and acceleration is good. Very solid feel. It gets around 19-20 MPG in the city. I don't really think that the interior plastics and switchgear are miles ahead of the Vibe and I personally like the Vibe seat fabric better. I also personally feel it's harder to get a car seat in and out of the Maxx than the Vibe, but I wouldn't qualify it as "difficult" and it may depend on the seat.
I also agree with Capriceman that if you're tall, you'll probably want the Maxx. My wife and I are both short, but I think it could be a real pain to find a good driving position if you were tall as the steering wheel placement is sort of odd.
So, I'd drive both and get what you like.