I like both - I group ride about twice a month, it is a good way for me to push my limits - I pick a group which is challenging, sometimes I ride that last 5 miles alone (not because I was the fastest...). I also ride with two friends quite often on short training rides - always the same course. Then I ride alone, sometimes for training and sometimes just for fun. Moral of the story - group rides can serve a purpose for you - but don't expect the experience to be the same as riding alone.