Choosing study destination and course

I am basically a graduate of commerce. In addition to this I have completed a two-year software engineering course at NIIT and a hardware course at IIHT. Also, I am a Microsoft Certified Professional. I have totally 6 years of experience in the software support field. I want to pursue my studies and wish to complete a Masters course in one of the following areas:
1. Computer Science
2. Networking
3. Information Security

I am applying to US as well as Australian universities.

My questions are:
1. Which is a better study and employment destination, US or Australia?
2. Which is a better study option in terms of opportunities and later growth — Networking, Comp Sc or Info Security?