They are very different, trangia are reliable and quite versatile in reality. Compact and impossible to get wrong. Kelly kettles are a lot bigger and you either have to carry wood or wood pellets or find wood when you camp. It’s hard to compare a campervan with a mini. They aren’t the same.
Trangia is easy to light and quite quick. It comes with everything you need to cook a meal. Kelly kettle just boils water but its advantage is fuel availability. You do have to buy extras (hobo stove kit) to cook food. Price wise, no real difference, ease of use the Trangia, novelty wise – Kelly Kettle wins.
Where to begin, two very different systems to compare, Kelly kettle is more of a camping when I’m on a campsite with family or fairly close to the car as it isn’t small, trangia when I need a compact system that I can bung in my pack, but to be honest I just use the jetboil more than anything these days.