I'm using a MIFI right now as a trial, and it will probably go back. It seems as though the MIFI has a hard time running and charging at the same time. It uses more juice than it can recharge itself with. This is a big issue for me.
I have both and im cancelling my mifi. Both are good. Down side is that you cannot use data and voice at the same time but doesn't bother me. I rarely use my mifi.
As far as speed, both are the same speeds, neither is faster then the other.
Mifi battery life is okay, but wick does use power and mifi has a small battery too. But overall, not needed for the light users. If you are on the phone a lot and internet a lot but without wifi anywhere else, then mifi but if not, no need.