Depends on the application, sounds like you're after MTB forks? If so, 15mm is pretty standard, 20mm are rather outdated.
20mm are rather outdated
But also inherently superior to the 15mm system. If the fork you want is available in either format, go for 20mm, but don't compromise your fork just to get 20mm; it's better, but not that much better.
@Bearlegged started
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Depends on the application, sounds like you're after MTB forks? If so, 15mm is pretty standard, 20mm are rather outdated.