With the edible printer, you have to buy edible printing sheets, the ink, sometimes replacement heads. With the projector all you need is the surface you are copying to (cookie/cake), and then your icing or edible pens.
When you're doing a ton of cookies that need the same pic/logo, the printer is very worthwhile. You do need to maintain it, esp if you use it only occasionally by cleaning the heads. The projector is very handy for odds and ends copying, and I love love love it for centering words. No maintenance required except for dusting and bulb replacement.
I have both, but if I had to choose one over the other, it would definitely be the projector!