I have the HD25-II and listen to primarily metal. For the money, I think they are pretty fantastic. They have punchy sound that just works great with metal. Haven't heard the ATH-M50, but they seem to get a lot of good feedback around here. A couple of things to consider:
Isolation: You mentioned being concerned about others being able to hear you. If you plan on using headphones in environments where there is an appreciable amount of noise, you probably want to block that out as well. If either of these two situations apply to you, you probably want to look for closed cans. The 598, 600, and 650 are all open (as are most higher-end cans). The HD25-II is closed.
Comfort and Fit: This can vary from person to person, but as a general rule, circumaural (around the ear) are more comfortable the supra-aural (on the ear). The 650, for example, is incredibly comfortable, while the HD25-II can start to hurt your ears after a while. Also, if you plan on using these while moving around, you probably want something that grips fairly tightly, but if you're pretty much sitting down the whole time, it won't matter much. (Random: I got my sister a pair of HD25-II, and she literally ran a half marathon wearing them. Those suckers have a death grip on your head, for better or worse).
Amplification: You don't mention an amp, so I'm assuming you're running yours from a mobile device or soundcard, correct? If so, you probably want to stay away from the harder-to-drive cans for now (HD600 and 650 are both 300 Ohms, so harder to drive; the HD598 is 50 Ohms and the HD25-II is 70 Ohms, so easier to drive). Of course, if you scored a good deal on some headphones, you could look into some of the more cost-effective amplifiers (Fiio and iBasso both have some good stuff) to properly drive them.
Other: There are a bunch of other things that may or may not be important to you (looks, compatibility with mobile phones, carrying cases and other accessories, durability, etc), and you just need to figure out what those things are.
I will throw one other headphone out there for your to consider: the V-Moda M80. In comparison to the HD25-II, I like them both roughly equally, although they do sound a bit different. I'll say that the HD25-II isolates better than darn near anything, but that the M80 is much more comfortable, and is MUCH better thought-out ergonomically than pretty much any other device on the market (gorgeous design, customizable, compatible with mobile devices, comes with a great carrying case, mil-spec toughness, etc etc). I can say that I've picked up no less than 4 HD25s and 3 M80s in the past as gifts for others, and everyone has absolute LOVED them both, but both of the people I got the HD25-II and the M80 for generally prefer the M80 due to all the extras.
Anyway, I hope this helps and doesn't just confuse you further. It looks like all the headphones you're looking at are pretty well-regarded, so I don't think you're going to be disappointed-- just make sure you get one that fits your lifestyle and usage.
Best of luck, man!! Let us know if you have any other questions!!