I prefer solo centuries (65 last year) to group centuries (3 last year not including brevets) - hard to explain why. Maybe I'm more anti-social than I think I am - I like silence, listening to music and starting/stopping when I feel like it - not when someone else does - is that anti-social or just self-centered? Brevets (200km and longer) are a different animal --- I like riding @ my own pace for 200 or 300km ones (though I'll ride with others on brevets) but would want to ride with others for any distance past that. But 100 miles? Shrug - that's a walk in the park and unless I'm feeling particularly socially-needy that day, I prefef to ride alone.

There IS a definite benefit to having others along if something unforeseen happens, of course. But I'm mechanically self-sufficient enough that I'll get most basic failutes fixed and get home without anyone else's help. So 100 miles and major malfunctions doesn't enter into the equation very heavily for me.