I had a 5150 II (6505+) for years and loved it, and one of the maps i tried it against when i bought it was the 5150 (6505).
Neither has great cleans, but the mark II's are slightly better, and the seperate EQ's is worth it in the long run i reckon. From what my ears told me, the mark II had a bit more mid range too, and the gain was smoother, the mark I's sounded a bit fizzy to me.
All in all, the 5150 II seemed like a fine tuned 5150, but they'll both give you brutal distortion!!!!
Hope that's helped