There's good and bad with both places, but I think that Coral Reef is the "most romantic" between the two choices. Here's why:
Both San Angel and Coral Reef can get loud; but if you eat early, you can have less of a noise at Coral Reef. That isn't the case at San Angel because there's a steady stream of visitors that comes to ride the ride.
Here's another thing: At San Angel Inn, the tables are really close together. You're REALLY close to other patrons. Uncomfortably close (IMO).
Coral Reef feels normal.
But San Angel is dark and pretty. Coral Reef has this giant fish tank. It's nice, but San Angel is pretty.
Can I say this? Because there's kids all around and it's the typical family vacation atmosphere, it's difficult to have a "Romantic" dinner. And if it is, you may want to go outside of the
DDP restaurants. Victoria and Albert's may be the "Most Romantic" because it's really geared towards adults. Go and spend that big grip of money and get the Chef's Table Dinner.
But for DDP dinners? It's pretty dark and campy at Sci-Fi Dine-In Theatre. You're in the car, you're watching previews of the funniest, weirdest B and C movies of the 50s. It can be romantic, with some imagination.