There was a time when the ACT (based in Iowa City) was "preferred" by Midwest, Western institutions and SAT (Princeton, NJ based) was "preferred" by Eastern schools. That has long gone by the board, and including USNA, the "preferred" test is the one(s) on which a candidate can score her best.
At USNA, a 570 is the cut-off at which a key official must intervene for requesting special consideration for a candidate.
The ACT is a bit shorter in exam time period, and is considered an "achievement" test. The SAT is considered an "aptitude" test, and unlike the ACT, penalizes incorrect answers. The conceptual difference is that the ACT measures achievement to date, projecting that future performance is best predicted by past performance. Aptitude purports to measure one's capacity for succeeding in the future. Who knows which is more valid in terms of its alleged objective, and who cares. The only thing that really matters is which, if either, does a candidate do better on.
Again, the USNA has no preference for which tests are submitted as long as they are submitted within the designated time period.