Reactors are mainly for quite big tanks. You can get inline reactors which you connect to your external filter and another type which you just connect a pump too and place the reactor in the aquarium but they arent the smallest of things and would be best in a bigger tank were you could hide it.
Get a rhinox 2000 diffuser and just place it under your filter outlet to blow the bubbles around the tank. I have a rhinox 2000 in my 200 litre tank and its brilliant.
id say a inline reactor is better than a diffuser, because it is outside of the tank, and it doesnt need cleaning as often, although i would use a diffuser over a in-tank reactor because they are just too ugly and bulky, IMO i would go for anything that you can put 'inline' with your external filter, anything out of the tank makes maintenance a lot easier plus it looks a lot better.