This may be unfounded, but here's how I see it. Over the years, the vast majority of razors have been 5/8 or thereabouts. There has to be a reason for that. Back in the old days when that's all there was to shave with, 5/8 must have been the favorite because there were more of that size made than any other. Of course there are the odd W & B meat choppers and such, but most of them weren't that big. I have 4/8 all the way up to 8/8, and while I like each of them for their different personalities, I'd have to say probably 5/8 if asked to choose one size for all eternity. The bigger ones don't shave me any better than the small ones, but I think the 4/8 are simply too light. 6/8 are good, but I find them a little more cumbersome under the nose. 7/8 and 8/8....maybe if I kept a moustache and didn't have to deal with the upper lip. As the big spendur said, sharp is sharp is sharp. After that it's purely aesthetics and comfort in the hand. Some 5/8 razors have heavy blades, and some 6/8 razors are fairly light, so there's spine thickness to consider too. I like a bit of weight to it, but prefer the 5/8 width when all is said and done. They're just less trouble under the nose. Add to the scale the fact that everybody and their brother is chasing the wider ones ~ driving up their value ~ and the 5/8 size is looking more and more attractive. This is just one guys opinion, however. BTW, I have a heavy beard, if it matters.
Jeff