No preference. Both are accepted and super-scored. But cannot take one score from SAT and the other from ACT.
THE BEST TEST is the one on which you can score the highest. Take them as often as time, money, perceived "best" results will allow. Be sensitive to timing of your application, its review, and the test and USNA deadlines. Plan waaaay ahead.
There are numerous conversion scales on the internet.
These tests are often perceived as "the same." They are not. SAT is an aptitude exam and ACT is an achievement test. In theory they measure different characteristics, the first potential, the 2nd achievement to date.
Just my opinion, but my anecdotal observation is that high performing, over-achieving students tend to do better on ACT. And also, you might well get as many opinions about this as there are posters. That's why it's important to try both if necessary, desirable to improve scores. Also, while dated, a few seasons back, it was acknowledged that the math standardized score was the most heavily weighted component of the scholastic review. ACT requires less testing time, about 30 minutes last I knew.
There is no value nor assessment made on subject matter SAT tests.