We just did this switch. Had a 2007 Honda Odyssey. It got 19 MPG. I loved it to drive it (DVD player, great for the kids, power sliding doors) but my husband HATED it. He is 6'2" and was very uncomfortable in it. His knees were up in the dash. Plus he's a mechanic and he hated working on it as it sat so low to the ground and everything in the engine compartment is so tight. I never rode in the van - always drove. While pregnant with my 3rd son, I rode in it a few times. I couldn't believe how bad it rode! You felt a lot of the road and it had a lot of road noise. Also, I always worried about it in an accident due to the way the nose was. I was in an accident in it a couple of years ago. Because of the slope of the hood, the Jeep that I hit was sitting up on the hood of the van. A little more force and it would have been in the windshield.
Anyway, we traded it for a 2010 Suburban (also my dream car). LOVE IT!! I'm a truck person anyway, but we love the way it drives, the way it rides, the comfort of the seats and I feel so much safer in it. We have 3 kids in car seats (7 (in a booster), 4, and 4 months). We have done both configurations - all 3 in the middle row and 2 in the middle, one in the back. No problem either way. We stuck with all 3 in the middle row because my 7 year old (who was in the back) wants to sit next to his baby brother.
As far as gas mileage - I was getting 19 in the van. The Suburban gets 16.2. Yes, it's an impact because prices are so high, but it's really not that much of a difference.
The only downside of the Suburban that I see is the seatbelt configurations. For car seats, the LATCH system is only two seats. And one of those seats is the middle row seat that is separate so you can fold it up for rear seat entry - so you can't put a car seat there if you want to use the third row. Also, the seatbelts in the back row do not lock if you pull them all the way out. They do lock based on force - so if you hit the brakes or are in an accident they will lock, but that doesn't help me with the car seats.
Good luck with your choice! If you have any questions about the Suburban, I would be happy to answer them.