I have flown both the C172 and the DA-20, as well as Pipers and Beechcraft. This is what I can tell you from my experiences:
B2N2 is completely right, cheapest isn't always the best. You get what you pay for. Sure, everyone wants to get a good deal. Sometimes people over pay in the end when they thought they were going to get a good deal. The school you choose is very important.
Regarding C172 vs DA-20, I like both but for different reasons.
C172 has more room so you can bring a big bag. Also, you can recover pretty easily from some mistakes such as being too high because it drops like a rock. On hot days, I like being able to open the window and stick my elbow out like a car. On a hot day, you can open the door during taxi to improve airflow.
The DA-20 glides really well, so you have to be more precise on energy management. It is also quite aerodynamic, so a forward slip to landing isn't as effective as on a C172. You really have to manage the energy and descents. I like the view because it has a nice canopy. When it is hot out, you have a tiny window, and you can't really open the canopy to taxi. I like flying the stick
Overall, I wouldn't pick a school based on the airplane, but rather the qualify of instruction, maintenance, etc...