well. I could ask this question million of times, there is simply no answer, I just got the money for high end engines and need one of them.. It has to be a good one, but people has 100 different opinions.. I know the C5 and C6 is completely out of its mind, so I think I'll go with the C6. I've seen that engine in action (C5) as well as the RB WS7II. Certainly, RB is the best engines you could get. I've come to the point where its easy to control and make fast corners with my present engine and thats why I need better
Also the P5 21-5 should have a quite smooth powerband, but the choice of engine is only a matter of taste imo.