Difference b/w Viator and booking directly with the tour co?

[Beachbabyboo]

Hi all!  My husband Sean and I (Cindy) are sailing this cruise and looking so forward to it.  First time ever for us to the British Isles!   I have used Cruise Critic boards before when doing  research for past cruises (such a a wealth of information and so helpful), but never participated in a Roll Call.  If we can be added that would be great!  We are from Dallas, TX.  

Had a few questions, what is the difference and is there a benefit to booking with a tour company directly, or Viator?  One example is the Jacobite Sensation tour https://www.jacobite.co.uk/tours/sensation and Viator’s Loch Ness Cruise with visit to castle and exhibition center https://www.viator.com/tours/The-Scottish-Highlands/Loch-Ness-Cruise-Including-Urquhart-Castle-and-Loch-Ness-Centre-and-Exhibition/d768-6570SEN.  They both appear to be the same excursion, although Viator doesn’t mention Jacobite in their description although people mentioned Jacobite in their reviews.  It appears Viator is about $5 more USD, but also says free cancellation, while I can’t find a cancellation policy on Jacobite’s site.  Lastly, would there still be foreign transaction fees applied to our credit card if we booked through Viator?  I assume there will be for sure if we booked through Jacobite.

There is so much info and so many sites I’m trying to wrap my head around it all!  Thanks everyone!