Choice of beach near Tokyo

I am going to Tokyo in late July, and want to make a day-trip to a beach in case of a fine day. I have narrowed the selection down to two possible places:
Onjuku on Boso-hanto
Zushi, close to Kamakura.

According to what I've heard, Onjuku is nicer than Zushi (cleaner, nicer atmosphere). The problem is that the express train ride makes Onjuku a lot more expensive than going to Zushi.
So my question is: Has anyone visited any of or both of these places (or any other nearby beaches)? Is Boso hanto so much more pleasant that it justifies the longer and more expensive train ride?

It'll be much, much more crowded in Shonan. Maybe you could base your choice on that. But the crowd can add to the overall party atmosphere. Just don't go in the water--doesn't look too inviting. For that, you can go to a pool--Tokyo has lots--try Yoyogi Stadium.