I have 3 iPad (iPad 1, Ipad3 and iPad Mini) and do all my navigating with it (do not carry a paper map.

Software wise I use Hema Maps with the NSW Topo Maps (not cheap) as well as MudMaps (which I hardly use) also not cheap
+ MotionX GPX (mainly for recording the tracks)

I usually have the close up on the mini and the overview on the iPad 3.

If I only would to use one iPad I would definitely go for an iPad three and not for mini. I would also not be too concerned about new models coming out as the iPad 3 will last you for long time for what you want to do with it.

I still have an iPad one for the kids and as a backup and even that does the job however quite a bit slower.

With around 20.000k's off road have never gotten lost and it saved the bacon a couple of times when we had to back track our steps.

I find none of the available software perfect (but I'm pretty fussy) and all of them missing functionality in my book. I really tried to like mud maps but always find myself going back to HEMA Maps For my general navigation but often use mud maps to confirm tracks. With these combination I have been travelling quite well. You could also use memory maps which basically is the same software and Hema just licensed it.

The only really negative thing is the price I spend easy $600 on maps and software which is quite steep.