The coffee bun from Roti Mum had the more distinguishing coffee aroma. The smell of coffee wafts from the bun unlike DJ's from which I didn't smell much of the coffee. You wouldn't know that it's a coffee bun if not for the label.
DJ's is more 'siksik' - kinda like a small mamon and it tasted bland. It was nothing special, it was just like an ordinary bread with a dollop of coffee icing on top. It had this cream filling in the bun which I guess was coffee creme but there wasn't any hint of coffee at all when I tasted it. I may be wrong about the flavor of the filling though :)
So for this round, Roti Mum's coffee bun won hands down. I've read that Kopi Roti's coffee bun is also good. Will probably try it sometime :)