I am very torn on this issue too and have gone back and forth.
The positives of using FP for Soarin and RP TT:
The walk to TT is easier, especially if you have a stroller which parking is right outside of entrance vs closer to Living Seas at Soarin and holding kids down stairs etc.
More people tend to go towards Soarin which gives you a chance to ride TT twice with little wait
You can always go back to TT later and do Single Rider
You can do Mission Space and/or Sum of all Thrills with little wait after your 1st run at TT.
Put your Soarin FP in around lunch and once you do TT, MS, Innovations, you can eat lunch at Sunshine Seasons before/after FP
Then spend after lunch doing Nemo FP, Turtle FP, Imagination, Captain EO
The positives of doing FP for TT and RP Soarin
Soarin's line will always be long, no single rider option, and the only way to do it twice without a FP is to go at RP and do it back to back.
The FP line can take over 20 minutes. It just loads slow
If you want to ride both first thing in AM without FP, you need to RP Soarin and get to TT after. It should still be between 20-30min.
TT goes down and closes a lot more than Soarin. Having a FP secures you a future ride that day when it opens.
TT can also open late due to water/dew on the tracks. People have posted going over and waiting 20+min before the first ride goes off. That would be frustrating
Kids love the exit games/cars with TT and if you RP it, you will be wanting to get to something else right away. Soarin has a quick exit