I ride faster on my own if not harder. I love going at my pace and not having to worry about waiting for people. That said I like the challenge of playing keep-up with the better/faster riders.
What I don't like is rides that are stop-and-start, which is usually associated with group rides. The before-and-after banter are great on a group ride, but as a parent of young kiddies I don't really get chance for the post-ride pint and chat.
On the road bike I definitely prefer riding by myself. Peace, solitude and reflection and totally, totally relaxing.
So, er, a bit of both for me then 😀