I haven't heard much about the Fairfield Inn, but it looks fairly new. The walk from the Fairfield in is slightly farther than the Double Tree, about 5 minutes more.
The reason why there may be conflicting reports about the Double Tree, is because it's one of the closest hotels to the parks, along with the Holiday Inn. With both hotels being that close, they are always booked, and sometimes they slack off.
I've since heard that the Holiday Inn has revamped a wing, and they know people are complaining about prior visits. Not sure about the current status of the Double Tree.
If you have kids and plan on walking over, the Double Tree may be the better choice since it is about 5 minutes closer. I wouldn't count on the hotel shuttles, they usually make you pay, and they only travel a few times a day. You'd be better off waking up early, and walking over to the parks at 07:30am. Tell the kids you're going on an adventure, then they won't complain about the walk.