I'm not sure you can objectively quantify the tradeoff between the additional items you find and the increase in difficulty, especially since it also depends a lot on how fast you are and what you will actually find.
However, I tend to always go to the Boar Beach if I can (without necessarily clearing the entire Ancient Valley first), because the beach itself is quite useful to farm:
- You get the experience and gold from the enemies, which are fairly easy to beat.
- You get the White Undershirt.
- If you've got Infusion, definitely go to the beach, because it's a nice quick boost to your total health, given the amount of rather weak enemies.
Also note that the beach itself doesn't really take very long to clear, probably not more than a minute.
As for whether it's worth clearing the valley first, I'm not entirely sure. I find it's one of the more annoying levels, so I'd probably rather go the beach straight away rather than doing the valley twice, but that's merely personal preference.