I would like to bring up what Kimmyt said about affecting the legs. Breast stroke stresses the inner knee and thigh. Fatiguing this area will predipose you to problems on the run, potentially.
Your slower front crawl times are likely related to body position in the water. I would guess that you're legs are hanging down in the water and creating drag and that you may be swimming "flat" (with very little roll side to side). Working with a coach even one or two times can make a huge difference and you may notice an increase in your speed very quickly.
So I am not in favor of swimming the whole event with breast stroke, but switching it up would be OK.
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