Get a Galaxy S4.
The S4 can be so versatile for anything you could want it for. It's more powerful, and has more apps, and a longer battery life. It's cheaper, so if you're looking to put it on a mobile plan, it will certainly be cheaper than anything Apple puts on the market.
There's a lot of stereotype about Samsung/Android being bad, because Android is what they call "open source".
But if the S4 has outsold the iPhone 5, it shows that the Samsung is, to the public, better.
The iPhone 5 is ok, but it's basically the overpriced 4s with another row of apps. If you're looking at screen size, the S4 pwns the iPhone 5. In terms of screen resolution, Samsung pwns iPhone's "retina display".
You really can't customise the iPhone that well either. You need to jailbreak it, which voids your warranty, so if the pretty aluminium back shatters, as it easily can, you're in trouble. The S4, on the other hand, uses Corning Gorilla Glass instead of normal glass, and a strengthened alloy of plastic on it's back, meaning it won't crack easily.
About viruses, Android has some of the best antivirus security on the market, including Avast! mobile security.
In conclusion:
If you're looking to make a lot of calls, and that's it, get an S4.
If you're looking to look cool, get an iPhone.
If you're looking to play games, get an S4.
If you want to save money, get an S4.