-More vesratile than Charizard
-Actual defensive power
-Better DW Ability when you combine Swords Dance, Growth and Synthesis
-Charizard's attacking stats: 84/109 vs Venusaur's 82/100. Hardly what I'd call a gamebreaking difference.
-Better boosting moves
-A reliable recovery move that doesn't require it to be transferred from Gen IV
-Leech Seed, Sleep Powder
-Better Speed under the sun
-No SR weakness and he absorbs T-Spikes upon entry
-No 4x weakness of any kind
Venusaur isn't outclassed by anything because there's just so much he can be. He can be a physical sweeper, a special sweeper, a physical tank, a special tank, a mix sweeper, an anti-lead, a subseeder, a staller, a stallbreaker...
All Charizard can do is sweep from all sides.
Venusaur got the better DW ability simply for the fact that it doesn't drain his HP, further dwindling his staying power. Charizard's staying power is already shaky when taking SR, Life Orb recoil and Belly Drum into account. Throw in the SP recoil and all it really takes is a decent unresisted hit to kill him. Or priority, either one works. Have I mentioned that Venusaur resists almost every single priority move in the game?
Combine Venusaur's notable Speed increase with Chlorophyll with his access to Growth which, under the sun, works like a Swords Dance and a Nasty Plot boost combined, and his ability to heal twice as much HP with Synthesis and you definitely have the better sun sweeper here.
Even outside the sun, Venusaur is useful as an anti-lead, a subseeder and a tank, plus he's still a notable sweeper even without the boost given to him by sunlight. Outside the sun, Charizard loses any significant advantage it had over other Fire types and just becomes outclassed. Venusaur's sheer versatility can keep him relevant even when he gets outclassed in one role simply because he can easily take up another.
In conclusion: Venusaur > Charizard.
And that's not even getting into design preference.