There is actually no difference between one form of Taijiquan from another. In essence, the underlying principles of Sun, Wu, Yang etc., have been founded based on the same set of requirements. The difference is in the characteristic martial applications unique to each style. For instance, the Chen style is known for its aggressiveness and is more focused on attack, and the attack is swift. Whereas, the Yang Style is considered more "gentlemanly" and is more defence oriented. Whether one plays the attack or defensive mode, both rely on the same Taijiquan precepts like, guarding your central equilibrium, maintaining your Peng Jing, etc. Both are just as effective.
It boils down to your personal preference of which style you prefer, and if you can find a genuine teacher! Whichever style you eventually learn is immaterial. What will set you apart from the mediocre will be proper guidance, your self-interest, and your persistence in keeping up your practices. Also you need to have a strong determination to excel in what you learn. Finally, a good listening ear is very important, especially when the teacher explains movements, its usage, and how a line of force should originate and end.