Keep in mind this is just a personal opinion... Either will do fine IF they are truly shave ready

but

A 6/8 is a bit heavier and that sometimes helps with those first tentative shaving attempts
The 6/8 can also help with keep the blade flat while stropping when you are new, and later on a 6/8 can be easier on the hones for the same reason


Again this is JMHO and either will do fine...