From everything I've heard and read, it's best to stay away from cities with container ports. That's where the cartel activity is highest. Acapulco is one of those cities. While you might go there and see absolutely nothing to cause you alarm, I wouldn't take the risk - especially in a group of 20 partying on spring break.
Puerto Vallarta is safe. Cabo, while there have been a few disturbing incidents in the last year, is also still considered safe.
There's plenty of night life here. We have great resorts, and it is very easy to travel around the city.
Twenty is a big group. If you all want to be in the same place at the same time, you might want to look at the larger resorts that will be better able to handle that many people around the pool and in the restaurants.