I was on Allure when she was new and it was may favorite ship. Recent reviews noted she is showing her age as she missed her dry dock scheduled during COVID. I believe she is getting the amplification done in January (might be wrong on that). So if sailing after her Amplification I would pick Allure.
However... I've been on Mariner twice and had a blast each time. Once before kids and last year with kids. Mariner is a great ship.
Pick the the price and sail date that works best for you and you can't go wrong with either ship.