Hope you don't mind if I chime in - I drive a Sienna but it came down to the Acadia and Sienna when we were car shopping, too. I was sold on the Acadia until we looked at the Siennas.
We went with the Sienna because we were able to get a lot more bang for our buck and the cargo space was better when the 3rd row was up in the Sienna. I also found the 3rd row to be more spacious/comfy in the Sienna and we knew that we would have adults in the back row at times so that was a consideration for us.
I LOVE the Sienna and I'm so very happy that's what we went with. My almost 5-year old can open his door and hop in the van without me worrying that he's going to slam a door into a car parked next to us. If my hands are full, all I have to do is touch a door handle and it automatically unlocks...no fishing around for keys. We ended up with a DVD player/wireless headphones, built in navigation, reverse camera, dual sun roofs (this may be standard though) and park assist.
Good luck - they are both very nice cars!
We bought an Acadia a little over a year ago anticipating our growing family. I absolutely love it- #2 will be here anytime now (hopefully) so we have had some time with just the three of us in it.
I went with the captains chairs in the middle, I preferred that since we were only having two, and it allowed easier access to the third row without disturbing the middle seats (where the car seats are). The middle seats slide forward and back increasing the leg room for either the 2nd or 3rd row. We have them pushed all the way back right now to allow for the car seats in the middle without having to adjust the front seats at all, and there is still a lot of leg room in the 3rd row (something I don't see a lot with 3rd row seats). And with the 3rd row up, there's still a good amount of storage in the far back, I currently have my infant seat w/base attached back there (ready for the hospital) and put my groceries in there this weekend with no issues. Both 2nd and 3rd row seats lay down flat which gives you a ton of space if you need to move a bunch of stuff. The front is really roomy, I know some cars still seem somewhat cramped on the inside but it's great. We have taken it on a lot of road trips and have never had an issue with comfort/space. I wasn't aware- but size wise it's actually longer than a Tahoe, so it's bigger than people realize.
As far as options go, I got the premium cloth (would have liked leather but we had a budget to stick to) which has been really nice- the front seats have seat warmers which has been a plus during this pregnancy, it was also a plus in Chicago but we moved to Texas and don't need it as much here. The remote start was also a lifesaver in both climates- and the remote tailgate is also awesome when you've got your hands full.
My only complaint has been the gas mileage, it says 17-24 mpg, but I have yet to get over 21 on road trips. I have a 30 mile commute both ways to work in traffic so I have just conceded to 17 mpg and move on.
Anyway- I went from a Honda Civic to the Acadia and I can't say enough good things about it, I'm not a fan of mini-vans (personal preference) so when I was looking at cars it was either the Equinox (smaller version of the Acadia/Traverse) or potentially a Tahoe. I knew we would outgrow the Equinox and Tahoe's are overpriced...
I should add more...
I have sat in both the 2nd and 3rd row and had plenty of room. It's easy for me to get to the 3rd row to snap a carseat when I need to, not as easy for my bigger husband. There is plenty of space in the "trunk" for the dog, but the backseat splits to fold down too which is a nice option. Gas mileage does stink!