I think Roserade is better. You can see the stat differences here:
-Neither can take physical hits well, but Roserade can take Special hits while Victreebel can't.
-Roserade has higher Speed.
-Although Roserade has less HP, the higher Sp.Def can cover that a bit. Victreebel has higher HP but doesn't have good defenses.
-Roserade has higher Sp. Atk. While Victreebel can function as a mixed attacker, I prefer just investing in one attack stat to power up. With Victreebel, you can either let a large offensive stat go to waste or use a lot of EVs powering up that stat for a mixed attacker.
So, if you want a mixed attacker, go with Victreebel. However, Roserade serves a better purpose for taking Special hits and dealing Special damage.
I hope I helped!