From what I understand, the 6505 series is no different than the 5150 series for the most part. The I has less gain than the II but really you don't need it. There's also an additional channel (at least on the 5150II) which is convenient for live situations. Not to speak for the Sneapster, but he'd probably tell you he prefers the 5150 I over the II because of the additional high frequency fizz you get out of it. You'll be happy with any of these, I'm sure.
These are excellent amplifiers but if you are playing at a low tuning (B or lower) you may consider spending more for a mesa rectofier or a krank amp as they sometimes handle low guitar notes a bit better, with more clarity.