The reason for the traditional leaning design in toolrests is twofold: effectively shorter distance to the work is possible, and a known tilt allows grinds designed to be sweetest steadied at that fully-supported angle. Loved my old Delta rest(s), so deplored the stock one on my 3000, which featured a knife edge behind the leading - flat top. Took a file to the stinker to get rid of the edge, and one horrible ding near the center courtesy of the former owner, and have lived with it. Would prefer a uniform slope, but have modified my peeling grinds slightly to allow use at 90 degrees, and changed toolrest height a bit from my old habit when cutting.
A round rest keeps you farther from the work, offers no limit in tilt. I don't like 'em as well as the tilted or flat iron, though I suppose you could modify your technique and grinds to use anything you had to. Also provides less contact area on the cylindrical gouges. Effectively point contact between the round surfaces, rather than along a bit of length. Transitions back and forth might lead to problems for simple folk like me.
I always ask folks who talk about dings in the rest the same question - what cut begins with the tool off the rest? Iron's fine, if you keep the tool in contact with it. Don't need hard steel.