My suggestion is to test drive them to see what you like. All of the mini-vans drive different (we just bought a Honda Pilot last weekend, but I test drove the minivans, the Highlander, and the Enclave). I thought I was going to like the Sienna the best, but the seats were stiff and I didn't like where I sat in the vehicle (I felt like I was driving the boat). I liked the Odyssey ride and the seats were more comfortable. The Pilot was redesigned last year and has a lot of space. If you boys get along it may be a good alternative as you can fit 3 car seats in the 2nd row (there is also a LATCH system for one seat in the 3rd row). But it is an SUV, rides like a truck more than a car, and has less cargo space than an van. The Enclave I liked as well (I think I liked the seating arrangement the best of all the SUVs), but I have hesitation about buying a Buick (or a Chrysler for that matter). We have a great mechanic (we had a Jeep, made by Chrysler, and it was in the shop A LOT after it hit 50,000 miles.) Basically he said he would buy a Toyota, Honda then Kia before buying any American made model (this is due to amount of things that break and the COST of the parts that break). So this also went into our consideration.
AND they are going to start making a updated version of the Nissan Quest for 2011 (so dealerships should have them in the Sept/Oct time frame). Which, from what I know of friends who have Quests, they love them.
Good Luck and just go one day and test drive them, take your car seats and hook them in to see how easy (or hard) they are to install and decide what you are going to love :-)