These examples (pairs actually) are from visit to an old power plant museum, where in low light I used my normal flash (Metz 58). Lucky I also had my tripod with me because the available light in many cases was way more interesting than what I could get with flash. Flash may reveal more in documentary sense but artistically I preferred the available light most of the time.
What do you think (a is available light and b is flash version):
In these my point is clear, I think:
#1 a
#1 b
In these one can argue for either one, which do you prefer
#2 a
#2 b
This is my favourite, but only with available light...
#3 a only