I had 6xx and 660s together before so I AB-ed them. I couldn't find much difference, nothing that justifies the cost. Also, I didn't had any amp to drive them to full potential that time. Sold 6xx, felt remorse, bought it again used, then sold 660s.

Then I bought 600 to scratch the itch, AB-ed with 6xx. Similar experience as with 660s. They are more similar but I prefered 600's brightness. Plus I had a dongle dac and balanced cables that time (ibasso dc04) to make a better comparison.

If you are on the fence, go with the cheapest one or the one whose look you like most. They are all similar than different with 6xx and 660s being tad darker. I think if I have to buy one later, I will go for 560s which surprised me during my short interaction with its price to performance ratio.