OzRunways (apparently about to produce a UkRunways?) have a bl0g with a discussion on iPad sizes
here. You'll have to edit the link where I substituted a 0 for o in bl0g
My only concern would be battery life if you don't have an external power source. My iPad 3 is down to 20-30% after 4 hours flying, in normal use it won't go below 70% in a day (non OzRunways use). SWMBO has an iPad Mini which has nowhere near the battery life in everyday use, 20% at the end of the day is normal. Translating that to OzRunways use wouldn't see me expecting much from the mini unless I had a power source to supplement the battery.
And my iPad 3 battery takes forever to recharge after a day of flying, too