I fly both the 172SP (G1000) and the DA40-TDi. The DA40 is a far nicer (easier?) aircraft to fly and students generally get to grips with landings more quickly. The go-faster-go-slower level isn't a problem.
Personally, I think the 172 is an average trainer. It has a number of flaws, not least being rather brutish when trying to teach some of the subtleties. As Neptunus says, lookout, especially in the circuit is a PITA. However, bearing in mind you are setting up a business, do not underestimate the influence that support will make. Everyone knows that you can get a Cessna/Piper fixed at almost any airfield across the world and you are never too far from reasonably priced spare parts. Diamonds are a little bit more tricky.