if you found hd660s good refurbished for 179,- (new 660s around 374,-~, mad) and hd560s new for 149,- , which one would be worth to get as the only headphone?
This is a tough one, I'd say it depends on what you are primarily going to use it for, if gaming is important, then HD560s may be the better option, otherwise I'd take the HD660s. You can very easily add a boom-mic to the HD560s by searching "hd598 boom mic" on amazon.
I have both, HD560s is great for the price, my one main complaint is that the pads are shallow and my ears hit the notch in the cups which is uncomfortable, seems like an easy fix, but most alternative pads that are thicker are also much denser foam, and this makes the cans far too muddy IMO. I have found a solution, the Accessory House velour pads, they are thicker, and are standard foam pads so are not dense, as such they sound very similar to the stock sound, but are thick enough that my ear doesn't touch the notch. The velour on them isn't quite as nice quality as the original pads, but its still good and I haven't felt them to be itchy yet.
The HD660s are built much better, they feel nice and premium, the pads are much stiffer, but there is no notch to contend with, and they mould quickly enough to become very comfortable, only downside in terms of comfort is the headband can create hotspots, where as the HD560s has a more forgiving head band.
In terms of sound signature, I'm no expert, but I feel the 660s are a lot more laid back, obviously a much warmer sound, especially with vocals. So as I said, if you want them primarily for gaming, then the 560s is probably a better option, but if you're looking for something primarily for listening to music then the 660s would be a better option, if you want something general purpose, then you really can't go wrong with either.