I've never been to Bice, but we had an amazing meal at Mama Dellas last year. The food was delicious, the atmosphere was relaxed and really nice. There were people playing music and serenading each table with the song of their choice. It was our anniversary and they even gave us a free dessert. We found the service good too. We made a reservation (though it wasn't too busy) and got in right away. The hotel itself is beautiful too.
Mama Dellas is good, but not much better than Italian you can find most places. Bice is a cut above, a truly exquisite Italian fine dining experience. The food is outstanding, and the service is impeccable without being stuffy. It's pricey, but great for an adult date night or special occasion.