I am currently doing Arts/Law at UNSW. I've been to both unis.
USYD does have many social opportunities, at least as much as UNSW. As someone mentioned Subski organises parties often. I would also say the clubs and societies have a bigger scene at USYD. However I would say that UNSW is a closer-knit uni. Perhaps it is the smaller size. It also has a great deal more food places to eat (and gather) than USYD.
As for the academic side, UNSW is academically focused. Both law faculties are about the same. USYD does have a traditionally higher reputation. This is not currently warranted, but it is a fact. UNSW has more law subjects however - you do double what USYD does in Contracts, Criminal Law, Litigation and Property Law.