I'm not too familiar with the NA X5 engines. I just went off with what was said in the OP. But like others have said, the V8 is different and will have better passing power. For regular day to day commuting and driving, the turbo 6 has been well to me. Do I regret not getting the turbo v8? Sure, but I'm glad I didn't when I see all these problems. I think someone said the S63 in the X5M is better than the N63 in terms of reliability. Again, no experience with them, just what I've read.