While Test Track's line is longer, I'll counter the common perception and suggest you get your Fast Pass for Soarin. Here's why I recommend that.
First, most suggestions to FP Test Track encourage trying to do Soarin at rope drop. While that works, it's my belief that rope drop at Epcot is a huge waste of time. Attractions where rope drop benefits you are just too far apart (Frozen, Soarin, Test Track). Compare that with a rope drop at MK where you could conceivably do Jungle Cruise, Magic Carpets, Pirates, Big Thunder, and Splash in under an hour. I've done it.
Secondly, Test Track is subject to break down and/or be impacted by the weather. Those are obviously things you can't plan for 60 days in advance. And, while you can always go back to Test Track once it's back running, that may not fit into your schedule for the day. With three theaters, Soarin is almost guaranteed to be operating at your scheduled FP time.
Finally, Test Track is best ridden by getting into line 5-10 minutes before closing. The wait times at the end of the day are inflated. We got in line with a posted 40 minute wait last October at 8:55 and ended up only waiting 15 (and that included building our car). The added benefit is Test Track is a better attraction in the dark. The time of day you ride Soarin does not matter at all.