[Jebus]

Personally, since the Genuine plugs are about 25 quid from your dealer I would just stick with them, at least the spec is spot on for the engine, but it is just a spark plug at the end of the day and as long as you get the recommended plug by the spark plug manufacturers they are going to all perform in the same way, some might give a better idle, others might be better at high revs etc etc etc

It is the same as the what oil to use, what filter to use and so on try to stay away from the cheapest, make sure it meets the spec and you are good to go.