Is there a significant difference in comfort going Business?

Slow ferries or sardine cans?

The difference is negligible, especially for such a short trip.

My 2 choices are:
FAST FERRIES THUNDER Dep 20 Jun. 09:50 (1 h & 55 min) Arr 20 Jun. 2024 11:45
or
SEA JETS CHAMPION JET 3 Dep 09:20 (01h 30m) Arr 10:50
Any thoughts?

As above. Both are big catamarans
https://www.seajets.com/learn-about-seajets/fleet
https://fastferries.com.gr/en/fleet/thunder/

There are no major differences when it comes to priority or privacy. So, if there's a big difference in cost, don't think about it.

personally I prefer the fast ferries. Who wants to spend 6-8 hours on a ferry if it can be done in 3-4? Don’t know what people have against the catamaran fast ferries. A bunch of sissies is what I say. The Aegean is usually relatively calm, except in rougher weather, which can happen in summer but more likely in colder weather. Come to the eastern seaboard and take a ferry to one of the islands off Cape Cod in 25 mph winds and heavy seas and you’ll see what suffering is.

take the Seajets. Done it a few times and have never had an issue.