That depends on your team core or what type of team it is, even when based off on movesets and stats. However, in general, Volcarona is better. Volcarona has only 3 weaknesses and 6 resists in which one is a 4x resist whereas Frosmoth has 4 weaknesses, two of them being 4x weaknesses, and only 3 resists.
As for stats and abilities, Frosmoth's Ice Scales ability is nice, but most Pokémon in the metagame are physical attackers. Additionally, the speed stat is not good in any way and its base stat adds to a total of 475.
Volcarona, on the other hand, has great speed that is higher than Frosmoth's by a significant 35 points (100). Plus, the SpA is higher. Both abilities are actually useful, with Flame Body sometimes making physical attackers more hesitant to attack and Swarm allowing it to hit harder. Each of Volcarona's stats are higher than Frosmoth's except for attack which neither needs.
When it comes to movesets, Volcarona's is much better than Frosmoth's mediocre moveset. There are only 8 special moves with a base power above 70, 3 of which are inaccurate. Only one of its moves counter a weakness, being Giga Drain against the Rock types.
Volcarona, on the other hand, has a large variety of great moves, with Giga Drain covering its Water- and Rock-type weakness leaving it with only 1 uncovered weakness being Flying. Its signature move has a 50% chance to raise its SpA by 1, (Fiery Dance) which is very useful and makes it capable of sweeping. Plus, it counters common Pokémon in the metagame, notably Ferrothorn, Corviknight and Rillaboom.
Hope I helped!