What's better group or single ride?

Depends on the group.  Riding with stronger riders usually ends up forcing you to push beyond the limits you know are possible.  Riding solo ensures that you can ride at the goals that you are targeting, but rarely lends itself to pushing those barriers as far as you would in a group.

I find that my best results come when I incorporate both types of rides on a regular basis.  Riding solo also makes sure that you don't push too hard all the time.  I know that if I do hard group rides too often, I'll end up overdoing it and can't sustain it week after week.

Couple of years ago when I was training hard I would do a small group ride (< 10 people) on the weekends that was ~ 50-60 miles round trip depending on the wind.  Always had a break at the half way point and pace goal without wind impacts was nearly the same 23-26 mph depending on who was pulling, but if someone was about to be dropped the pace would back off.  At the time I raced sprints at 23mph and had no issues riding with the group.  What was fun was they had certain points in the ride were there would be "king of the hill" sprints, always coming up to the halfway points, and depending on the route coming into a town setting or a bridge crossing.
I definitely benefited from riding with them as my focus at the time was for Sprint racing with mostly VO2 and Threshold workouts on the trainer.  Going longer with this group allowed me to do long Sweet-spot riding with some pull intervals and sprinting mixed in.  First few rides I would be exhausted/sore and became stronger.
Mind you, were I live is one of the worst areas in the Florida for pedestrians/bike riders and have an understanding with my wife to only ride with others and not alone outdoors.