I enjoy both.
Solitude at speed on a Welsh country road in the quiet of the early morning is priceless.
When I'm alone, my main problem is the company I'm keeping (ha ha ha ha).
But seriously... I was alarmed recently to be riding with friends who were heavily tilted towards RLJ-type activities. Lovely people, but I found myself getting all Victor Meldrew about it.
So pick your companions with care. Even really good pals might be poor cycling companions.
Also, some people are intolerant of those less fit than themselves. This is rare, but can be a pain.
Others are smug about their higher fitness levels and will rub it in by making a big song and dance about having to wait.
My fave riding companion (other than my children) is a sports coach friend who is significantly faster (and knows it) and significantly more knowledgeable (and knows it). He never rubs in his obvious edge over me. He is always encouraging and often comes out with helpful tips about cadence, breathing, recovery techniques and similar.
Cycling is his 'thing' and he isn't smug about his knowledge or ability.
Also, if you're going to be doing biggish miles with two or three riders on each other's wheels, you need to have a lot of trust. I've never ridden in a big group, but even three bikes drafting at speed is a potential snarl-up.
As to gear, I like lycra but try to avoid visible branding and team colours. By coincidence some of my clothing matches - but I never wear matching items together. Really, that makes me a sad as the folk who insist on wearing matching gear, but I'm in denial about that last point.