75% introvert, living alone was much better for productivity and happiness because I was better able to regulate "interruptions" like hanging out with friends etc., by planning when I went out and when I came back.

I lived for a while with 4 roommates and that was terrible for productivity, as there was a constant background noise of talking or music (I need silence for concentration, so maybe not applicable if you don't), which sometimes went very late into the night. It was a positive for happiness however, as we lived in a "hip" part of town and everyone got along fairly well, so there was a lot of opportunity for quality social experiences.

Currently I live with my girlfriend and a close friend. For productivity it's been pretty good, but mainly because my girlfriend is also a graduate student so it's easier for me to concentrate at night if she also needs to do so. Living with a close friend is bad for productivity because if you're both home at the same time you'll inevitably spend some time talking or hanging out. I think if my girlfriend was not also in graduate school this would be a very bad setup for productivity. Happiness however is great in this configuration as I'm often near two people I care a lot about, who are both respectful and responsible. It's the quality social experience of the second setup where I had a lot of roommates, without the unpredictability of chatter and music late into the night. In other words, it's for the most part a pretty good average between living alone and living with a group.