aren't you paying practically the same as an iPhone plan? the 10 a month for the voip service plus cellular service for the iPad mini?
I find VOIP works better on Android than iOS.
Problem is that VOIP apps can't run in the background for very long. It gets put to sleep by iOS, so you aren't always connected.
So expect many missed calls. Also expect the occasional drop calls and distorted low quality calls.
Although I won't give up my Nexus 4 for calls, I did set up my Nexus 7 (with cellular) to make calls using Groove IP. I tried it while I was out driving on T-Mobile and the audio was all broken up. It's fine on Wifi just sitting here. It uses Google voice to take and make calls and receive text messages. Just wish it was better while moving in the car.