Depends on whether you want open headphones or not. If you are going to be using them outside, or your listening environment is noisy, I would probably go for closed. I prefer open headphones wherever possible, myself.
I have tried the ATH-M50 (non X, but still the same sound). They are supposed to be good value, but I wasn't overly impressed. The AD500x do look mighty interesting, though. I have good experience with Sennheiser, and from them, you have the HD 558 in that price range: https
/en-us.sennheiser.com/audio-headphones-high-end-hd-558. The AD500x are still lighter on your head, mind you.