My DD rode both when she was age 3. Soarin, we told her she would be flying like Peter Pan, and she absolutely loved that-- although at the end of the ride, she looked at me like I didn't have a brain in my head and said "That was like Diego, not Peter Pan, we were in a glider!"). At that age she loved them both and wanted to ride Test Track again, but decided she did not like TT the 2nd time through. At age 4, she loved Soarin, but was scared of TT and didn't really like it. Took another year or two before she really liked TT again.
Now that you have to choose one or the other to FP, I would say Soarin. But, that said, it really kind of depends on your child. Soarin is definitely the tamer ride of the two-it's a smooth sail. TT is jerky and bouncy and speedy.