Well thats it, they are more similar than different. But also some like a bit bloat! The mids bass hump of the 650's is very nice, but imo not natural. The 600's defo do electric guitar better as they also have a more natural and detailed treble. The 650's roll off at the top more, smoothing a little too much in comparison.
This is obviously just my opinion but I lived with the 650's for years (6 to be exact) and loved them. But once you get used to better transparency and balance its hard to keep appreciating them as much. Both are great headphones but the HD600 are for me - the audiophiles choice, over the more fun and relaxing 650's.