The Sennheiser ones, are really good have more bass than the Audio technicas, however lack a little in mids and lows not as much, but just a little. The Audio-technicas apparently have amazing balance between mids, highs and lows, their bass doesn't compete with the U 320s, but have a really good balance. It lacks a mic, however it also seems they are more comfortable than the U 320 and have a more durable build. U320 = $99 and the ATH-M30X = $109, so the 518's are $20 more than the M30X.
I would say yes the $20 extra is worth it then. If you want a more balanced sound signature then I suggest you look at the Sony MDR 7506 or Shure 440 with 840 pads.