Test Track is a far more popular ride - if you get in during early entry, I would get FastPass for Mission Space and head directly to Test Track. Am not sure how much difference it will make - I can tell you that during busy times, FastPass for Test Track can be gone by noon or 1 pm for the rest of the day.
Guess it may not make a big difference - especially since they located right next to each other. We've done Test Track first and then gone to Mission Space and not had a really long wait - that was in October/November...