Are there any advantages of using SailMail versus WinLink over HF? I can't say I'm significantly familiar with either system, but at first glance they appear to offer similar functionality, with of course the latter requiring an Amateur Radio license.
Any cuttings in data transfer have a huge return in less costs and faster replies.
The few options out there are focused on email and expensive. To get an idea: sailmail.com offers email-only SSB (single side band) for $250 / year.