You may have made the decision already, but Spa day definitely. Sadly, dine out is buggy, and seems to break as often as it's fixed. I think a nice replacement for dine out is get together's cafes, if you really want somewhere for your sims to catch up and eat. There's also lounges and bars etc - not the same, but I feel I use those a lot more than dine out. If you really want dine out, I'd recommend waiting until it's half price.
I also feel spa day adds a lot of cas options that are useful too - while there is fitness stuff/discover uni, I still use a lot of Spa Day's outfits. Whereas with dine out, it's a little more dated now imo, including the build/buy. The BB for spa day is better designed I think, especially the kitchen counters. The wall/floor tiles are also something I struggle to build without, especially if I'm designing public bathrooms, and the wood swatches are lovely.
Gameplay in general is also a lot more solid in spa day. There's multiple items and also a few hidden surprises like the mood-modifying incense sticks and the drinks tray.
