OP
If you do hill repeats by spinning up hills in your easiest gear, than you are probably not getting significant bang for the buck for them. Hill repeats are something that you repeat to failure, with the goal of fatiguing yourself. If you don't get there, then you aren't getting there.

If your goal is to be the fastest possible, you need aerobic strength and leg strength. Sometimes, it makes sense to mash, some times it makes sense to spin. On a long hill, spinners win, on short hills, mashers usually come out ahead. Be able to do both.