Windows or Linux?

[F1.z3ro]

Well arguably any linux comes in different flavors. But a base install of mint will give you a gnome environment

No, what I meant was you can DL mint with KDE as a default. 

[Dreamer]

linux really is best, so much is done for you on windows and even on linux there are less people that make programs for you which would turn you into a 'skid'
Linux takes some learning but it will make you better in my opinion.
there is always cons and pros.

[noob]

Dual boot:Ubuntu+Windows7

[lucid]

Or if you really want to learn about linux then dual boot Arch + Win7

[sudo_arachnid]

Id say linux, but thats just me, since linux is open source they have a lot of great communities that will help learn all about how a comp works, and what you need to know.

[Anon-frost]

It all depends on your personal opinion. I actually have 3 operating systems installed on different partions of my hard drive and I use each one for different reasons. I am in a situation where no one can know I'm a hacker especially because of who I'm involved with so I usually have a second copy of windows setup as a cover OS.

[Phage]

Something is clear if you want to hack computers you can't get past Windows. I will also recommend using windows as your main OS and Linux on a VM or from a flash drive. Do that until you get familiar with it, and then swift to Linux as your Primary OS.