We booked through DL.com for our June 2011 trip and GAT for our December 2011 trip.
Booking through DL was good for our first trip as we were able to get 0% financing using our Chase Disney Card, free gifts, the tickets were plastic and easier to locate and keep track of. Our booking also included a personal phone call from Mickey. The video of my son answering the phone is priceless. I think we got a lot more communication from DL (emails, post cards, etc).
We booked through GAT in December because it was last minute trip and DL.com didn't have the 4th night free for the hotel we wanted. GAT was about $137 less which imo was a significant difference. We did have to wait in line to get our real tickets at the ticket booth which meant an extra line first thing in the morning. We didn't have any issues getting the tickets. The CM was great and had all our information. The second day an agent from GAT called us to make sure we arrived safely, that our accommodations were what we requested, etc. I thought this level of customer service was great.
My only advise is that I wish I wouldn't have pre-purchased our character meals from GAT. We upgraded our tix to AP's and would have paid less w/ the AP discount than the pre-pay discount.