Flash or f2.8

I never use flash except for macro. Just not my style of shooting. I'm just like that and love the look of well-composed photos that make use of available light. Sometimes that involves asking your subjects to move so that said available light shines nicely on them, but I don't mind doing that.

If you're going to get a prime for low-light purposes, I'd say f/2.8 is too slow for dim indoor shooting (if you're talking about churches and romantic restaurants and whatnot ...)

I'd suggest getting at LEAST a f/1.8 (nifty fifty costs $100). Or better a f/1.4 ($300ish) if you can afford it. It makes a hell of a lot of difference.

And as a bonus, blurring out backgrounds generally goes very far for indoor composition since interior backgrounds tend to be full of distracting junk, on average.