Your instinct of Scald is a better one in my opinion for the following reasons:
1) the difference in power between Surf and Scald is rather small. Surf with 90 power and Scald with 80 power
2) You mentioned you have another Pokémon learning Surf thus making it redundant to teach Blastoise Surf.
3) The added effect of 30% chance of burning your opponent further assists Blastoise with its usual role as a mixed tank.
Source: Bulbapedia, (Surf and Scald pages) and Pokémon Database