The 5150 and the 6505 are identical. The 5150 II and the 6505+ are identical.
The first year 5150s were block-letter EVH after that it was Eddie's signature. The block-letters are supposed to be better - but they were the same design - only they came with better stock tubes. If you get a block-letter that still has original tubes, they probably need changing.
The 5150/6505 vs. the 5150II/6505+, the II/+ has....
Separate eq and presence/resonance per channel
Footswitchable crunch.
Voiced a little brighter/thinner, cleaner clean channel (not saying much really)
More usable gain on tap (less saturated than the 5150/6505)
1 extra preamp tube