I love both the round holes and square holes. I do not have a Cinch. I use a different very old binding machine to make round holes.
Square holes I use the Bind-it-All. The BIA can be a bit hard to use, a little bit of a learning curve and you do it the wrong way and get pinched. I like to call it the Pinch-It-All.
I love bookbinding and I adore Smash books. For most of my smash books I like making round holes. I feel it gives more room. Once you start Smashing your books can get thick.
Square binding I like for journals. The kind of journals sold at Barnes & Noble. The square holes make for more elegant flat pages. I just love how cute mini books are with square holes.
Now, I don't want anyone coming in here and yelling at me saying "Square holes make great Smash Books!" "You are discriminating against square holes."
! I have seen a lot of pretty Smash books made with Square holes. I just prefer, for me, to have more room because once I start putting in my treasures in the book I get thick books. I can adjust sizes better with round holes. The square holes your type of coil you use can be limiting and hard to remove when you need to alter your book.
Hope that helps.