Welcome to the BBS,
I prefer Sat nights due to working during the week and there is often a rush to get prepared for a gig. Sat is after a Friday night chill time and the logistics can be managed a bit better by having more time to get to the job, set up, get a fix on the system and instruments, and still have chill time left with o rush. Also more folks that make up an audience also have jobs during the week and use Sat night is the Party night. At least thats the way it is around here where I live and work.
When I was gigging a lot, I prefered Friday AND Saturday in the same establishment. Only one setup and one breakdown for two days pay. For some reason it seems Friday nights were the bigger crowd though on average.
I second what Track Rat was saying. Although, most of the time, our gigs were usually two different places on Friday and Saturday. So we always had to setup and breakdown twice a weekend...