The best pad for a well fitting English saddle is a simple, thin pad, either full size (rectangular-ish) or fitted (shaped to the saddle) depending on personal preference. With a saddle that fits well, the pad is just there to protect the saddle from sweat and dirt. With a saddle that doesn't fit quite perfectly, some people opt to use specific types of pads to help improve the fit. This is where a half pad might come in. That being said, there are also trends in the riding world that some may follow regardless of true necessity for the individual horse and saddle combo, and then there are people that use certain types of pads because they've "heard it was good" or their friend/trainer/barn owner told them it was good, etc.