I have the GNS5780 and no complaints about its core function, but its ability to turn itself on and drain its battery is becoming a real pain. I went flying on Friday and it had been clipped, face down, against my kneepad in the flightbag.
Even so it had managed to turn itself on. I can always run it off the DC outlet, but it is annoying.
If I was buying another iPad I would deffo go for the 3G/GPS model. My iPhone GPS never seems to fail in the air and most people report the same for the iPad.