You’ll get a very big difference for your $200 - in some parts of the US it would barely buy you a better view.

The MO rooms are relatively newly refurbished, whereas the Peninsula hardware is really verging into museum territory. The Oriental also has much better F&B options than the Peninsula - useful for a honeymoon.

That said, I’d be sure to check pricing for Capella or the Four Seasons (or the Siam if budget will allow it). All are riverside and in the top tier of bangkok hotels. I don’t especially enjoy the Oriental relative to alternatives in its price range, but that’s a bit of a matter of taste.