AvPlan or OzR run quite well in 16Gb, and we used to recommend that as a minimum. However people always put other junk on their devices, so get one with more storage (32, 64 GB) if you can.
Always choose one with cellular (for data) and the inbuilt GPS.
As for maxi-pad vs ipad mini, thats a very personal choice. Best is to get both in your aircraft and work out what suits you
Bevan..