My impression, based on playing the NES version on an emulator and seeing screenshots of the GameBoy version on deveria.com/alexis, is that the NES and GameBoy version are probably pretty much identical.
The NES version is not as bad as I thought. A0;Some of the graphics are even strikingly good for an NES. A0;But I definitely prefer the more responsive gameplay of the normal versions.
It can be somewhat more challenging than the normal versions, but for mostly bad reasons. A0;For example, when you pause the game, the only thing you can do is to move the cursor around, you can't even change the currently selected skill while pausing. A0;And skill assignments are not instantaneously, they almost seem location-based, meaning that once you assign a skill, the old skill seems to need to be carried out to a certain point before the new skill kicks in. A0;Oh, and the falling distance is shorter--Fun 3 for example the only survivable fall is off the lowest platform above the exit.