The 6XX essentially IS the 650. Drop doesn't always retune drivers with their variants. Some headphones they just use existing headphones, add their own aesthetic, and sell as is. The 6XX is one of those.
When they retune, they specifically state as such, and add graphs comparing base models to their retunes.
The only differences you can expect with the 6XX and 650 would be driver variation, and whether you heard a much older model of the 650 which was tuned different from the more recent 650s of maybe the last....8 or so years.
So unless you really want the 650 aesthetic, want its slightly different inclusions in the box (I think the cable is much longer on the 650), or live outside of Drop's shipping regions, I'd save my money and get the 6XX.
I personally think the 6XX looks better. No blue logo, for on the ends of the headband for one. And the dark blue looks black in most lighting conditions.