they're very different - i've had both. the nova system is way better for effects. so if you're looking to pair it with a good tube amp, it's way better than the G3. But the G3 works as a backup rig with pretty good amp modeling. the effects aren't as good, but it does more.
the G3 is easier to figure out and use and cheaper and it fits better on a board, but you don't want to count on it for drives. the drive on the nova system (which is analog) is pretty good.
standalone w/a tube amp, go with a nova system. part of a pedalboard w/other drive pedals or for modeling, go with the g3. i'd give the slight edge in overall satisfaction to the G3.