Originally Posted by
Bah Humbug
Comfort will be the kicker for 500k in 24 hours. If you are "normally" proportioned, go for whichever would be lighter while fitting you properly. If there is anything you dislike even slightly about the stock geometries you have easy access to, go custom. Twenty hours in, you'd give up a couple of pounds to be comfortable.
I don't know about 24 hours. My old Panasonic road bike would rattle my teeth out. I could roll over a pebble and it would feel like a bolder. I can ride my new bike all day long with out any fatigue. My attitude is ride as many bikes as you can and pick the one that feels right.