Good headset for gaming and general media

The main difference is that one is open back and the other is closed.

The Audio Technica ATH M50x is a closed back pair of headphones. This means they will isolate noise better and will not leak sound. They have a slightly v shaped sound signature, meaning the low end (bass) and highs (treble) will be emphasized. Very forward or powerful sounding. They use a removable cable, although it is proprietary. They can also fold up for portable use. They are low impedance and can be driven from a variety of devices without the need of an amplifier.

I would not pick them for gaming for two reasons:
a) not the most comfortable. Pleather ear pads which become hot and sweaty after a while of use... high clamping force, poor headband padding.
b) They have a small sound stage

Their sound is very similar to the ATH m50 and M40x. I have owned a pair of ATH M50s in the past.
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The ATH AD 500x is an open back pair. This means everyone around you will be able to hear what you are listening to, and you will be able to hear them as well. Unless you are listening at a high volume. They have a neutral sound signature and you will hear your music/audio how the producer recorded it. Open back headphones tend to be a little light on bass and the same would apply here.

These would make a good choice for gaming. They would be significantly less fatiguing the m50x both sound and comfort wise. It uses a 3D wing design for the headband and has breathable velour ear pads. They are also more detailed and will have an airy, wide sound stage. Footsteps would be easier to pick up and positioning should be great for gaming.
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However you should use a pair of headphones that you enjoy listening to and focus less on how good they will be for gaming. Really any headphone would work just fine, some have a slight advantage over the other due to their design. For rock and metal I would look into a pair of Grados. The SR60 or SR80 should be in your price range.