decat is just a pipe to replace the Cat, big increase in power but illegal and noisier, also will mess up your hego sensors so you need to bypass them to I think, lot of hassle. Cat back is full exhaust from the cat back ie retain your existing cat. Only a full system from manifold to tailpipe will give some genuine extra BHP, any other parts not too noticeable other than better sound.
K&N first then full system.