It doesn't matter too much. You will probably prefer one over the other when you get proficient, but at this point either is perfectly fine choice. 5/8 is the standard size. I'd estimate that 80% of all razors ever made are 5/8.
I prefer 6/8 though, 7/8 is already too big for my taste, and I have only found one 8/8 razor to be ergonomic enough (it's a livi in olive wood scales). I can get a nice shave out of any quality razor from 3/8 to 10/8 but 6/8 is simply the size that makes the experience most natural. The only way to know what works best for you is by trying out different options.