Kotthu Roti, is probably the most famous Sri Lankan Street food, almost everyone in Sri Lanka loves them. I too am a fan : P... Its always worth a try, there are many different varieties to choose from these days. As such I would certainly add trying Kotthu to your Itinerary for Sri Lanka!
If you are not too big on spicy, ask the place you are buying it if it can be mild. Watching Kotthu being made it self is an interesting thing to do.
Below is a video I found on YouTube which talks about Kotthu (locals usually call it Kotthu and dont alway use the Roti part).