Having stayed at both several times, I would take the Peninsula, which is lovely and has terrific service, over the PH anytime. The Peninsula is probably my favorite hotel in LA; the one thing I do not like about it is the pool, which gets terribly crowded, but the pool at the PH is relatively unappealing.
Of the other hotels suggested, forget the HR, which is convention-like and charmless. I was unimpressed with service at the BelAir, though the pool is very lovely. I don't find Santa Monica all that convenient unless you want to spend most of your time there (which you certainly could--LOTS of great restaurants) and I did not have a great stay at Shutters; I actually prefer Casa del Mar if you want to be there.
If it's any consolation, I don't think the FS there is quite up to par. I do like the Regent Beverly Wilshire a bit more.