Have to strongly disagree. Have both and have played through several others and I can definately say that one does not "destroy" the other.
They are very, very close to each other. Each kick for metal tones, and with the addition of a 2nd channel, I'd go with the 6505+. Both are awesome and do practically the same thing. The 5150 is a kin to an EMG where the 6505+ is a kin to a Blackout, both great for metal, fuck all between them.
And I have to strongly disagree with you. Unless you're using a crappy cab, or guitar, or what very well might be the case in this instance, active pickups that kill any warmth in your tone, they sound quite different.
The 5150 II/6505+, while having an improved clean channel, is muddier on the lead channel. It is smoother and much less aggressive, which tends to make it a better lead amp. It responds very nicely to a set of good PASSIVE pickups, which will give you a cleans tone that's surprisingly good, especially for the 5150 series. They also have a bit less gain... Like that's really an issue with these amps though
The 5150/6505 has a clean channel that is completely useable, if you know what you're doing. Don't expect the best cleans out of it though, especially using actives. The lead channel is much more aggressive and clear with more high end sizzle which makes it cut through a mix like a knife.
The best, and clearest leads come from boosting the crunch setting on either amp though. The bias mod also improves these amps so much, that I still can't believe Peavey hasn't added it as a stock piece over the years. Sure, the 6505+ has a bias pot, but the sweep on it is so tiny that even with the knob all the way up, it's still not running the tubes at a voltage anywhere close to where they should be.
P.S. As you can probably tell, i dont like EMGs. I know this forum seems to really like them, but I personally am not a fan; they are sterile and lifeless and just do nothing for me. The Blackouts are noticeably better, but still not anything I'd waste my money on. It really gave me a good chuckle the other day when I read a post here where everyone was praising EMG and saying that brands like Bareknuckle were inferior.