I like solo - i think it allows you to really get to know yourself better, and I tend to be more aware of my surroundings when doing something solo. At the same time, it definitely keeps you thinking more about the 'what if". But I think that's a good thing - it keeps one more on their toes and (for me at least) the ego in check.
I'd like to say I make the same decisions solo as I would if with someone, but that's simply not the case. I tend to be more cautious on solo missions (or conversely a bit more reckless on group missions), because that safety net of someone else there with you is gone.
However, I think group mentality tends to cause one to push themselves a bit harder, maybe because you realize you have somewhat of a safety net in case something does happen. I find that I tend to ski harder or ride harder when part of a group. That can be good and bad - good in that you may try something new that scares you (and conquer that line you've been scared to try), and bad that it might place you over your head in a situation where if you were on your own you may not be in the first place.
I've scared myself and hurt myself doing sports solo (broke a vertebra in my neck mountain biking by myself a couple years back - spooked me quite a bit in terms of the 'what if' factor), but still love the solitary feel of being out on your own. I still ride on my own quite a bit, but have learned to tone back on some of the more dangerous stuff. I tend to surf alone quite a bit as well - the calming effect of being in the water by myself is something I just can't explain.
I tend to dislike doing anything with big groups (say over 4). Call me antisocial or whatever, but in my experience, there's just too much going on with over 4 people. Anyway, I wrote more than I thought I would on this...just my $0.02...