Obviously this is theoretical, since it depends on the individual horse. I'm talking about as a whole, which type of horse do you prefer for dressage?
I find that the Spanish horse's compact build can often make bringing their hind under themselves a lot easier, and they dominate in the airs above ground. I don't see Spanish horses often in the WEG or Olympics (if at all, I am not a close follower), however seeing riders such as Oliveira and Phillipe use them is enough for me. Warmbloods, however, have more length in the gait and are typically easier/more common to find than a Spanish horse. They tend to be more flashy with their movement, but the leggier ones are harder to collect. They are much more prevalent in the competition ring.
If I had to choose blindly, I'd say Spanish horses, since they seem to master the higher levels of dressage (airs above ground) more frequently than Warmbloods. I own a Warmblood however because a) they are much easier to find than a Spanish horse, at least in my area. And b) I also want to do a bit of jumping, and Warmbloods are certainly better jumpers.