I am one of the people who can't choose. DO is essential to me, because as someone already said, there are not many places that make sense for your Sims to dress fancy and while I have a number of restaurants in all my saves, there is one that is a staple: Stargazers (formerly the Stargazer Lounge in San Myshuno). For some reason I don't get tired of my Sims having romantic dates there, especially to ask to start a relationship or propose and then take a picture together with the skyline in the background ...
But Spa Day is so good as well. Not only to have a Spa, but also for the functionality of just placing the instructor mat and a few yoga mats in any gym and the game start to schedule yoga classes there. Also, the BB is great, especially the tiles, I could not do without those. Oh, and it comes with a special wall and flooring that has a wonderful glow and looks great in pools! The only thing I am still upset about, is that there is no Wellness aspiration, but the Wellness skill itself comes with a couple of great recipes, the ability to influence other Sims' emotions and teleporting. So, my answer is: Wait for a sale and get both. :D