Which do you think would be best suited for longer audaxes like PBP, a low or high racer, or does it not matter?
Best suited for who? You, I take it. Forget anybody else.
Are you looking to ride PBP, or is your enquiry just a general one?
By some peoples standards [mine for one] PBP's a long way, 1200km to be exact - I'd look at ergonomics first and foremost, regardless of low, mid or high racer.
Me, I just like the 700/406 configuration. I don't like being too low and I don't like being too high. As a consequence, I have to take two different sized tubes which is a pain, but that's the price you pay.
You're going to be sitting on it for 4 days [if you're a full value rider like me], you want something that feels like it's your cup of tea from the outset.
The bike that's suited for PBP is the bike that's suited for you.