I've never even noticed this! Assumed you were talking about the SHA256 one. I'm still going to assume it's faster for all the same reasons minus the P/Invoke.
Don't forget that on WinXP (yeah lol) only the Managed one is supported. We use the following code in our systems:
var sha256provider = Environment.OSVersion.Version.Major >= 6 ? new SHA256CryptoServiceProvider() : SHA256.Create();