Douglas has much more of a selection, at least the stuff that matters to me, I can almost always find there. The guys at Douglas are always extra kind to me with hardware as well. I have left there frequently with hand fulls of grade 8/ 10 hardware for nothing. They do the same thing there.
Every time I have visited Fastenal I was not impressed with the extra small selection they carried, and there were plenty of times I left there empty handed, because well they didn’t have it.