Only thing that makes a big impact, and ain't all that big is the razor's edge, and they're not that much different if it's on a 9/8 or a 4/8, so other than getting used to the different "heft" in the hand, both will shave you closely or cut you the same.

I'm 6'4" 230lbs, love the big razors just as much as the small French framebacks...and been bitten by both.

If you're just starting off, 5/8 or 6/8 is a great place to start, once you get used to your first razor, going up or down the ladder won't be anything other than different and fun....