I say go for it.
I spent four days in San Salvador in 2001 (Sep 11-14), and neither I nor anyone on my crew had any problems walking around and exploring for hours on end.
My experience in Central America tells me that if you encounter any crime at all it will be minor, street-level crime (purse-snatching, mugging, etc). Nothing really dangerous. And this only happens in the worst areas and very rarely.
Go down there, help them out and get to see Central America up close. You’ll appreciate it for the rest of your life.
Chalk me up as another person who doesn’t think Guatemala is as bad as it has been described in this thread. I really liked it there.
If you go, El Salvador is going to be very different than you are what you are used to. If that sounds appealing you should go, but if deep down it is not appealing you shouldn’t.
I’ve seen a lot of travellers in countries create their own bad experience. They are too rigid in their expectations, or they are on high alert for the bad, they don’t see the good. If you go remember the entire country of El Salvador is not going to change for you.
For the OP, you have to ask yourself how tolerant you are with being in strange and challenging situations. Are you comfortable not always following a plan? Are you resourceful?