Hi,
I have both 5/8 and 6/8 razors-- actually kinda hard to tell the difference on the shave. I prefer slightly larger, as they hold more shaving cream before you need to rinse/wipe them off. I think the 6/8 size just looks better. 7/8 look too big and 4/8 look too small...

Bottom line, if you find a good razor you like, both of these sizes will give you a similar experience.

-Chief