It's a really, really tough call for me, as I use both of these packs all the time. That being said, Spa Day just barely gets an edge, because (unlike Dine Out), none of its objects, features, or locations have ever been the slightest bit buggy for me.
Also, I use the wellness skill on almost all of my sims. Mind you, I also take most of my sims to both restaurants and spas all the time, but... for everyday use and play on the home lot, the wellness skill is a little more versatile.
Honestly, though, I would find it difficult to play without either at this point.