If you buy the PH6 now, you should replace its stock pads, that's for sure, What kind of pad you should get depends on your personal comfort experience. If the stock pads seem comfortable enough, I'd risk going for the best sound quality with the low repulsion pads from final audio. If the stock pads are uncomfortable, try one of the 3rd party replacements people suggest in this thread.
I heard the HD650, but not enough for a real comparison. I remember a much darker sound than my PH6. The HD600 might be the better Sennheiser, at least for my taste.
When in doubt, you should probably try the cheaper ones first - the PH6 doubles the cost of your alternatives.