I'd much rather do 5x6h, I suppose the difference might be if you have small children.
But as someone without kids, being able to take short flights or trips every week over 3 days would be awesome.
I guess ideally, since 5x6h and 4x8h are only two hours apart, a utopian workplace would offer its employees either option -- the people who chose 4x8h would make 2h more wages weekly.
If I wanted, I could work 4x10 (With some sweet talking to a manager perhaps). I'd very much prefer to have 30, 32 or 35h to distribute over the week though, it would make the planning a lot easier. In that case I'd do e.g. 5x6 now that my small children needs early pickup, but I'd go to 4x8 when I have larger children who can bike home from school, so we can go on a weekend trip with friday off.
Of course, if everyone works 4 days a week and has the same 5th day off that advantage goes away I suppose.
Now that I have kids and work an office job, I would much prefer a 5x6 to a 4x8.
Why not? You'd get to spend more contiguous time together.