I believe the Hyper 212 EVO will be the better choice, I assume the N520 is based off of an older design whereas the EVO is newer and offers better performance. Most of use recommend the 212 EVO when on a budget and looking for the best cooler.
Edit: Forgot to mention the 212+ and EVO perform about the same, I believe the 212 EVO beats it by a few degrees (1-3 degrees, just a guess. They are pretty much the same).
They both have the same installation brackets, so the difficulty will be the same. The 212 is the better cooler, but it is tall, and might not fit in all cases. The N520 is shorter, and will fit into some of the narrower cases out there. If yo have a midtower case or smaller, you'll want to take some measurements to ensure the Hyper 212 will fit. If you have a full tower, you don't need to worry about it, just get the 212.