They are both OK grammatically, but they can be interpreted differently.
It is critical to EITHER RUN or FLY.
This means it is critical to do one of these things--we can choose one or the other.
It is critical EITHER to RUN or to FLY.
This could mean that either it is critical to run or it is critical to fly. So one is critical, and perhaps the other isn't, but we don't know which is which.
Dmitry Farber | Manhattan GMAT Instructor | New York
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