I did quite some research before I purchased my first straight razor and out of that research I concluded that the 5/8 would be the best option. So I spent a great deal (too much) money on a brand new semi exotic 5/8.
But as of 2 weeks or so ago I purchase a 6/8 from an antique shop for $39 and I find that to be my preferred shaver now.
Reflecting back I regret spending a large sum of money on my first razor, there was no need for this.
Like others have said you will most likely buy both at some stage anyway so it does not really matter on the size.
The 6/8 felt a little big in my hands at first but now it feels great/just right and the 5/8 a bit too fiddly and I have no problems at all with the 6/8 under my nose but maybe as I am a bit more skilled now.
Oh and don’t rush out and purchase an expensive strop either as you may cut it up, unless the leather is easily replaceable.
I would advice to purchase a shave ready razor off the classified on SRP or find a good quality at an antique shop/eBay and get it honed and wait until 6 months of straight razor shaving and reading the forums on SRP before buying something you really like the look of. Well I wish I had done that.