Purdue admits are harder to get comparatively. USC needs money so they admit people with low gre scores and undergrad scores.
You might find it easier to do well in the courses as the filtering is really bad at USC.
The masters students are called "cash cows".
Getting funding for masters is out of question
http://www-bcf.usc.edu/~dkempe/
check his work out and also the chair of cs ,ming de hwang etc
You need to work for atleast one to two semesters to consider you for phd
Purdue has some great people for security like Elisa Bertino etc
See the courses for both and decide.
USC MS cs general is not difficult at all.
