Having only been on Allure of the Seas, I was blown away with the art work, especially in the staircases, truly amazing...I was told the decor on Allure is nicer, but not having actually been on Oasis, who knows. I would choose the ship that works better with your travel plans, either leaving on Sat or Sunday could make a difference. Also, be sure to check prices and go with that. I also highly recommend using a cruise travel agent for booking. They may even have a group price if they have a group booking at the time you want to go.
Do let us know which one you pick...and what itinary, East or West.