Depends on how you plan on spending your day... If you're there all day and gonna make rope drop I'd do a late PM Soarin. You can knock out a couple runs on each at rope drop then enjoy the park, take advantage of your late FP+ and do single rider TT later in the day with little wait.
All other things being equal: FP Soarin. The stand by at TT is much more interesting than the screens at Soarin. And with the new pre show activities, a FP at TT still means 20 minutes or more to get to load area.
OMG Soarin! That queue (or lack thereof) will suck the life out of you. As has been mentioned, TT has the more interesting line, and has the single-rider option.
We did Soarin' for our first day which has us in EPCOT around 3ish (plus it is the DBF's favorite ride in all of WDW.) TT for our last day since we will be getting there at ropedrop (and I prefer it over Soarin' personally.)