I did the Culpeper Century last year, and it was great. It was a mass start event. We went off at 8:30. The first half hour we rode in a large group, as pretty much no one wanted to ride past Ted King (http://www.iamtedking.com/) who was there with a local racing club. The pace though was moderate, maybe 17 mph (I didn’t keep track). Ted and co. pulled off at the 1/2 hour mark or so, and then the pace picked up and the big group broke down into smaller groups riding at various paces. Anyway, even without Ted King and that dynamic, since it’s a mass start event if you ride your pace you’ll end up riding with other folks.
Culpeper is rolling hills. The Seagull is flat flat flat. Me, I’d rather do hills, but if 100 mis. is a long ride for you then Seagull might be better. But Seagull has thousands of riders, whereas Culpeper is small. Another reason I prefer Culpeper. (But I’ve never done Seagull.)