I think I will go with SidT's suggestion and get the 4G version but without the SIM card initially. This avoids the monthly costs but gives me built-in GPS. If I find the signal unreliable, I can always get an external GPS receiver later on.
I can then use my iPhone 5 for any data I need when Im out and about (likely to be on the ground rather than in the air though) - Out of interest, for those that do use data in the air, how useful is it?