Hi L. Tucker, and welcome to cruise-chat.com!
I don't think you can go wrong with either line. I cruised the Mexican Riviera on Diamond Princess a few years ago and had a great time. Although it was a large ship (as Sapphire Princess is), it very rarely felt crowded.
I've sailed a few times on HAL, but not on the class of ships to which Oosterdam belongs. Yet, as another fan of Celebrity, HAL was a good fit for me.
Which Celebrity ships have you been on, and what have you enjoyed about them? That may help us help you with your decision.
BTW - Rick, I also saw the Mariner Magazine and the cruises you mention. If only my husband wasn't a teacher...none of these dates work for us.