Hi there!
I have travelled around El Salvador a fair amount...down the coastal spots mostly and through the capital to neighbouring countries.
San Benito is a more upscale and safer area...used atm at night and walked to restaurants etc, however that may be a long way from where you intend to visit.
My experience is that there are always "safe" areas but having a big SLR would draw some attention...a phone less so and they take great pictures too.
The advice by c-w-s is, as usual, spot on!
One trip we left at night using the main bus terminal...we had arrived late and missed our connection...took a local bus to a colonial town outside the capital...believe me it was like something out of a bad movie...funny now though!