Okay, I'm not sure if you have already decided which one you are going to choose, but from experience I will tell you this:
Both Pokemon are very good, so I'm going to go over them one at a time.
First, Darmanitan. Darmanitan is mostly known for his incredible Attack (140). Other than that nothing else is that noteworthy, his HP is pretty good, and his Speed is average. Everything else is pretty poor, with his Special Attack, Special Defense and Defense way below 60. Overall, his ability and moveset aren't bad at all, and he ends up learning some great moves like Superpower, Overheat, Flare Blitz and Belly Drum, personally, I would say Darmanitan would be better for In-Game, as it helps you through many challenges in B,W,B2,W2.
Secondly, we have Volcarona. Volcarona has become a very popular Pokemon due to his spectacular Special Attack (135, not quite as good as Darmanitan's 140 but still very good!), and its great Special Defense, as well as about average Speed. Its Attack isn't nearly as good as Darmanitan's, but it's HP is getting close. Overall, Volcarona would work better as a Post-Game Pokemon. His stats do surpass Darmanitan's, however in game it is far harder to obtain Volcarona, as for Darmanitan you could either raise a Darumaka, or use the Rage Candy on a statue outside the Relic Castle. Volcarona also learns some very great moves throughout leveling up, such as Bug Buzz, Fiery Dance and Quiver Dance.
Both these Pokemon are very impressive, and both would make a great addition to a team. In the end, Volcarona stats do surpass Darmanitan's, however they both have decent abilities, and a good learnset. The choice between Darmanitan and Volcarona might even depend on how much you like Attack over Special Attack or vice versa.
Here is a link to a site that contains a comparison between the two Pokemon, as well as stats and their complete learnset.