That's okay-a lot of people prefer the 600s to the 650s. I have both, and the 600s accentuate the higher frequencies a bit more, so they sound a bit less 'veiled' . I'm not going to use the term 'dark' for the 650s, because I think that's an old wives tale, but the 600s sound could be construed as 'more revealing'.
At 200 bucks for the 6xx, this would be a good opportunity to try both and keep one without too much of a financial hit. Or keep both. (-:
The HD600s are better for classical and jazz. I once had both and kept the HD600 for that reason, because it's easier to find a headphone that can do more popular music well for less money. I will be getting the HD6XX though because they are worth it for the price, and I need new earpads for my 12 year old HD600s anyway.