I have the iPad 3 32GB Cellular, and just purchased an iPad mini 32GB Cellular. My previous experience was with an iPad 16GB WiFi only.
The iPad 16GB WiFI was sufficient for flying with reasonable cost, using Foreflight software. However, 1) I needed an external GPS antenna connected via bluetooth to have location services enabling the Foreflight software to track my progress in air (to know where I am); and 2) I needed to have access to a WiFi internet connection to take advantage of weather briefing, filing flight plans, nexrad, etc. No big problem, just a hassle for me. The 16 GB was sufficient as long as I didn't load up a lot of apps, data, documents, photographs, etc.
Having the 4G LTE cellular capability is really nice. I am connected to the internet just about anywhere I happen to be (except in the air above 1500 ft. AGL). Plus the internal GPS antenna is nice as well; you don't get that on a WiFi only unit.
Another consideration now days is with all the ADS-B antennas and features out there on the market today, you may find that the WiFi only is fine since you may opt for one of those devices like the Stratus, Clarity, or Garmin that also give you WAAS, a more precise location than what you get with the internal GPS antenna on the iPad cellular units.
I got the iPad mini because the iPad 3 takes up a lot of real estate in my 182. And my iPhone is hard on the eyes. So the mini fits just right. Small enough to not take up a lot of room, but large enough to see the maps and all the information you need.
So, my recommendation is to stick with your iPad mini for a while and then reassess after you have been flying for a while. It will work just fine for now. Look at all the different packages out there and you will find that most of them offer a 30 free demo trial. I tried them all and found Foreflight works best for me. Check out Fltplan.com. Their app is free!
You should have plenty of room on the 16GB as long as you don't store a lot of photo, videos, etc.