It all depends. What do you value most? I do work on my own pet projects alone, but I value being a member of a larger team, because by confronting your ideas with those of others you learn more and faster than if you just do things yourself. Also, since we are all humans and doing stupid things is in our nature, it might be more difficult to spot errors you do in design/architecture, and consequently more costly to fix. You also don't get the benefit of thorough, specialized testing - at least in my team there are specialized testers too.