Although it would be a closer match, Venusaur would still be the superior in my opinion. Why, you ask?
Venuasaur's hidden ability, which doubles speed when the [the sunlight turns harsh], in tandem with the move Growth, which doubles both it's Attack and Special Attack in harsh sunlight, makes Venusaur the undisputed king of sun sweepers, and arguably the most powerful of all non-legendary grass types, and it's usage reflects this. That stated, this Venusaur is quite different from its more defensive mega-evolution, being more of an offensive sweeper, and requires sun support when being used, which really hurts this generation as Drought no longer provides permanent sunlight, but the benefits are worth it.
A good set would look something like this
Venusaur (M) @ Life Orb
Role: Mixed Sunny Sweeper
EVs: 176 Atk / 128 SAtk / 204 Spd
Naughty Nature (+Atk, -SDef)
-Hidden Power [Fire]/Sludge Bomb
Courtesy of Josh's top answer [here]
Now, that being said, Roserade is by no means a bad Pokemon, and is actually only just edged out by Venusaur's superior stats and movepool. Roserade's speed and special attack are actually quite a bit higher than Venusaur lacking a sun boost. In addition, Roserade's access to Spikes and Toxic Spikes make it great at supporting the team, and it can serve as a good specially defensive wall or as offensive support, amking Roserade generally the better choice when not abusing the sun or mega-evolution.
A good set could look something like this
Roserade (F) @ Black Sludge
Ability: Natural Cure
EVs: 252 HP / 242 SDef / 16 Spd
IVs: 0 Atk
- Giga Drain
- Toxic Spikes
- Sleep Powder
Courtesy of Me
In short, Venusaur has the better movepool and is overall a better sweeper, but if you are not running a sun team or are looking for something that can serve a support role, then go with Roserade