I haven't been to either, but my friend Geoff has frequented both, and he prefers Gunnison. Don't know why, but perhaps because he is originally from the northeast!
Regarding San Onofre, Things are much better there now. Rangers do not appear to be ticketing this summer at all. They are short staffed , and with new State Parks Management, nudity may no longer be an issue there anymore. However, the beach at Trail 6 is small and still rocky. People still go south of the fence onto Marine Beach, but there is a relatively new "No Trespassing" sign put up on the fence between the State Park beach and the Marine Beach. And the Marines are enforcing it. My friend Dan was taking a nap in the nude on the marine beach when a Marine jeep came up to him. The Marine told him to get dressed, and then proceeded to write him a citation for trespassing on government property to the tune of $500!!! So don't ever take a chance and go down there.
In any case, Blacks Beach is a much better alternative for nude use-and the lifeguards are friendly and don't care if you are nude or not.