WiFi is WiFi, Data is Data.
The only thing you pay for on any mobile phone is minutes and Data. I only use my Data when I'm away from a WiFi source. I get unlimited Data which is throttled after 2GB (throttled means they reduce the download speed to the device until the new month, where you start from scratch on you plan). This mess's up nothing. 3G is slow. WiFi is faster, If I want to watch an old Star Trek episode on my phone I use WiFi so it's not choppy/laggy. If you have a VM MiFi and it is faster than the normal 3G, go with it.