That's a good question. Personally, I would trust the maps more, because they not only show you the route of the trail, but they also show you the side trails that can lead you to safety in the event of an emergency. GPS is good for making sure you're on the right path, but it doesn't always give complete information about the surrounding area.

You might not have to buy the entire set of maps -- if you're starting in the north in May, you might hike to Arad and save the Negev for another trip. In that case, you could get away with just buying the maps for the northern half of the trail. Then again, if you think you'll buy them all eventually anyway, it's cheaper to get the whole set.