Hello L. Tucker, and welcome from me also;
I don't know if you are a past-passenger with Holland America; I am, and I just yesterday received their "Mariner" magazine, and in the back under "Best cruise values" is an offer for just what you are asking about: 7 day Mexican Riviera, r/t from San Diego, 11/1 and 11/8/08 on Oosterdam for the incredible price of $499 inside OR outside and $699 for a Verandah. Sailings on 11/15, 11/29, 12/6 and 12/13 are $499 inside, $549 outside and $799 Verandah.
I'm booked on Sapphire Princess 10/11, and If I wasn't already into the penalty phase I would cancel and rebook with HAL.
I usually go with Princess because I believe it is a great value for the money; but I must tell you, from the one HAL cruise on which I've been, I have to say they are a cut above.
If you are not elgible for the discount, I would say it comes down to, would you rather sail from San Pedro or San Diego.
Sapphire Princess is my favorite of Princess' large ships; I love everything about it. But prices being equal, I would have to go with Oosterdam.