I would definitely go for the round one. If the only scraper you want to burnish is a rectangular one, you will be happy with the triangular burnisher but it feels less comfortable on round and especially narrower inside profiles. I bought the triangular one because one of the woodworkers from fine tools said he liked it more than the round one and since I had no experience at all I took it.
I’m using a 2nd cut file.
Also little trick put file in vise and run your scraper flat on bench top against file.Also if you have diamond plate put that in vise as well.
It’s easier than trying to do that than trying to hold file level and flat on scraper in vise.
I using round burnisher, you could use an un-platted screw driver of good quality carbon steel or Gouging chisel.
Guys, I have some thoughts on burnishers that might help. The best one of all is one I had made. I will let you know on my blog shortly what I discovered and even how to male one that really works.I would certainly not buy a triangular burnisher. I have found round or oval is best all rounder without compromise.