CaptainMongo
The 13 inch would be better because it has more real estate. In split screen, you could have a sort of reasonably sized approach chart on one side, and another app for whatever perf app or document reader on the other. The regular ipad is just too small for that.
To be clear, this is a limitation if we go paperless. if all you're using is one app at a time, the regular ipad works just fine.