Whew, okay. I've actually used all these programs now! (Spent some of the weekend poking around Blender and talking to some Blender experts.) I went to college for media arts and animation, and worked with 3DSMax and Maya for all 4 years there. I currently use 3DSMax in my day job. My conclusions? At basic functionality, there is NO difference between most of these programs, just in the way the interfaces are designed and how easy to use they are. The differences are only skin-deep. 3DSMax vs. Maya is pretty much a choice of "which one is easier for you to use". I like 3DSMax. Most of my graphics making friends prefer Maya. I've been using Max for about 10 years, they learned on Maya. We can all create the same quality work, we just have totally different workflow.
3DSMax: I use this at work, so of course it's my favorite. It's got a relatively easy to figure out interface (with a little time spent on the tutorials), but most of the advanced options are hidden in obscure menus. But hey, how often do you NEED stuff like occlusion mapping, global illumination, and mental ray rendering? (Not often, if you just build armor.) Commonly used for video-game quality graphics, MSRP $2000-$3000. There are thousands of tutorials, including a full set included with the program that can get you up to speed in a hurry.
Maya: Used this in college for a few years. It has an obtuse and convoluted interface (in my opinion), but once you know how to get around it, you can kick all sorts of butt. This is what most high-end graphics are currently created using. MSRP $1000-$5000, depending on version. The tutorials are adequate, and there are a lot more available online.
Blender: Although it's not a frontrunner in the industry, it's gained a grassroots and cult following for high end graphics applications, including games and special effects. And heck, it's FREE, don't complain. It has almost all of the features of 3DSMax and Maya, but the interface is a little harder to figure out if you've been trained in other 3D software. There's a growing community with tutorials, guides, and even 1 on 1 help available.
Gmax: This is essentially a stripped down gutted version of 3DSMax. But if you just need to make polygons and models, this is dang good for ease-of-use. It doesn't have the fancy options of Maya, Max, or Blender, but mostly you don't NEED all that extra stuff for what we usually do around here.
Conclusions: 3DSMax and Maya are awesome but expensive, Blender is a free equivalent (but slightly harder to use), Gmax is basic but does the poly editing just as well as 3DSMax, making it a good choice. If you just want to edit polygons for armor, I'd give Gmax a try, if you want more, try Blender. Or dish out thousands of dollars, but I know a few up-and-coming game developers using Blender now, and they are making VERY cool stuff with it.