In my corner of the world I have three clubs to choose from: the "Slow Spokes" -- name says it all. Relaxed, social, smallish groups. The Clinton River Riders (my club), pretty much a middle of the road outfit with rides from easy to pretty fast. And then the Wolverines, who count among their current members and alumni some top amateur and pro racers including some TDF and Olympic riders. Fast but still quite social, which surprised me. Also tend to be large groups. Doing 20mph+ thru a subdivision in a pack of 30+ riders can be, um, exhilirating if not a bit scary! I ride with all three depending on the pace and convenience. Of course, with the Wolverines I do mainly their "recovery" and "moderate" rated rides.
I ride with all three, depending on who has a ride at a pace I want on a night that's convenient.
Riding with a group, and the social interaction, are motivators for me. If I had to count on my motivation to ride solo I'd be a lard butt. I do ride solo once or twice a week in addition to 2-4 club rides per week.
Also, as often noted, riding with a group can push you to ride faster (even allowing for drafting) and farther than you might solo.
But it all depends on your personality.