I don't notice much of a difference between the two honestly. When I do dressage shows i'll wear the white full seats. I don't do dressage shows much though unless its a green horse I am working towards eventing.
Anyway yea. White breeches only for showing and then only if you are showing dressage or higher level jumpers or eventing. As for full seats... its a personal preference. Some people swear by them. Then you get those of us who don't notice a difference.
I will say though that when I had full seats and was riding in an old school wintec saddle that was the fabric seat, I stuck like glue! Lol