iphone

Wi-Fi is faster ( as long as you have fast internet) because it is a direct wireless signal straight from your internet connection via a wireless router.

3G is the cell phone companies' internet connection, slower than wi-fi, but still fast for mobile internet.

People nowadays prefer both so they can switch between them if they want faster internet at home. Wi-fi uses less battery of the phone but has the limited range of however far it reaches (which is usually about your whole house). Hope this helps you a bit.