A lot of what system I decide on using depends on how I want the game played. While the content of the game isn't system specific, how the game plays is very system specific. For instance, a game like 3.5 tends to be less deadly than say, Basic Role-Playing where a lucky critical hit can mean the end of a character. I'm being fairly general here, but on average, one system is much more deadly than another.
Another consideration is the type of game play each system gives you. Again, comparing 3.5 with something like Hero System, Hero tends to be less deadly and more 4 color comic book, with the main type of damage being stunning damage.
Now, my suggestion would be to look at the following systems, most of which have a test drive version of the rules available if you look around enough. Basic Role-Playing, Savage Worlds, All Flesh Must Be Eaten (don't believe there is a test drive for this one). Then also consider 3.5, but you already have experience with that one so you have something that you can compare it to.
Hope this helps.