Mifi is just a gimmick for phone company's to generate more revenue with more monthly fees.
Your cell will work flawlessly and save you the $15 per month fee for the Mifi.
I have one of the cheapest smart phones and use mine as a hot spot all the time. I can accept calls while using it and I have not noticed that much battery usage either. If concerned with battery use just hook up to your charger.
Also, make sure you have enough data on your plan as to not incur overage charges. I have 3 gigs with ATT and they doubled that for a total of 6 gigs with rollover at no extra charge. I pay $103 per month for 2 cells and unlimited talk, text, and 6 gigs of data. It's hard to beat that.
Walter and Janie Rogers
2012 Sundance 277RL
TV 2006 Silverado 2500 6.0