RE: Which one should I get?
It depends on what you're looking for. Here are the main qualities of each, IMO:
Slow and Heavy builds strength. It is a three-day split routine, which means you divide the body into three parts (chest/back, biceps/triceps, legs/shoulders). The workouts are built on three sets of an exercise for one body part, before moving on to the next exercise. One body part is finished before moving on to the next.
Push-Pull/Supersets offers two full-body workouts (as well as premixes that allow you to do just upper body or just lower body). It is good for endurance and some strength building. Push-pull is a superset workout (according to how the term is traditionally used) that works opposing muscle groups like Chest/back, one right after the other. This is a very efficient way to get more work done in less time, as you rest the chest, for example, while the back is working. The Superset workout is actually a circuit workout, where you do a series of exercises for different body parts without much rest in between.
Which one you choose will depend on what type of workout sounds more appealing to you, and also where you are in your weight training. If you're ready to take your strength up a notch, I recommend Slow and HEavy. If you don't want to spend three separate workouts to get the whole body worked, then SS/PP would be a better choice.
If all else fails, just flip a coin! (No matter what the results, you'll discover when you toss it what you want the results to be!)