Both Venusaur and Charizard are good sun sweepers, however Venusaur exceeds Charizard in that aspect
First of all Venusaur and Charizard are equally fast, however Charizard's special attack is a little bit higher then that of Venusaur. Both have access to incredible moves. However Charizard's main downfall should be it's rather bad typing that makes him have a 4X weakness to Rocks. Whereas Venusaur can easiliy kill of pretty much any Pokemon in sun
Venusaur is better in sun as with it's hidden ability, it get's a free speed boost+you have the chance to attach a great item to it, whereas Charizard needs a Scarf to be doing anything close to what Venusaur can do under sun. With Growth boosting both it's Attack as well as Special Attack I should really say Venusaur over Charizard in a sun team. It has a better coverage, a better ability and a very interesting movepool that will enable it to sweep through unprepared teams
