There is another option too. My iPhone is jailbroken to allow an app called MyWi which takes my 3G signal and turns my phone into a wifi router. Essentially the same thing as having a Mi-Fi, except that I have one device that does it all. Best of all, the app is $20 one time and the jailbreak is free, so no recurring monthly charges. If you are already paying for cell service, why pay for another device?
AND, if you have an iPad, there is another app that will allow it to automatically turn on the MyWi on the iPhone when you need an internet connection! It don't get no betta.