***Mega Venusaur vs. Celebi***
Celebi
Stats: 100 HP / 100 Atk / 100 Def / 100 SpAtk / 100 SpDef / 100 Spd
Weaknesses: 4x Bug, Fire, Ice, Dark, Ghost, Poison, Flying
Resistances: Grass, Fighting, Ground, Water, Electric, Psychic
Other Advantages: Use of Item, Natural Cure, Doesn't use Mega Evolution spot (if this is a problem)
Mega Venusaur
Stats: 80 HP / 100 Atk / 123 Def / 122 SpAtk / 120SpDef / 80 Spe
Weaknesses: Psychic, Flying
Resistances: 4x Grass, Fighting, Water, Electric, Fairy
Other Advantages: Immune to Poison, Removes Toxic Spikes from the field, Can still use Chlorophyll ability in the Sun before Mega Evolving
Now, using the laws of mathematics, let's cancel out all the common attributes in order to get right to the advantages and disadvantages of each Pokemon vis-a-vis the other.
***Comparison***
Celebi (compared to Mega Venusaur)
Stats: +20 HP, -23 Def, -22 SpAtk, -20 SpDef, +20 Spd
Weaknesses: 4x Bug, Fire, Ice, Dark, Ghost, Poison
Resistances: Ground, Psychic
Other Advantages: Use of Item, Natural Cure, Doesn't use Mega Evolution spot (if this is a problem)
Mega Venusaur
Stats: -20 HP, +23 Def, +22 SpAtk, +20 SpDef, -20 Spd
Weaknesses: Psychic
Resistances: Fairy
Other Advantages: Immune to Poison, Removes Toxic Spikes from the field, Can still use Chlorophyll ability in the Sun before Mega Evolving
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Obviously, this doesn't include the X-factor: Movepool.
Celebi is more diverse and unpredictable with what it can do. Heal Bell, U-turn, Nasty Plot, Calm Mind, Baton Pass, and Earth Power are some of the excellent moves Celebi can use that Venusaur does not have access to.
Venusaur does have one nifty potential trick up its sleeve though - the ever useful Sleep Powder. It's also more viable as a physical attacker with access to Petal Blizzard and Earthquake.
***Final Thoughts***
If you're already using another Mega Evolution on your team, then Celebi is obviously the way to go. Celebi is also going to be the far more diverse and unpredictable Pokemon. It's one of the best Utility Pokemon out there.
Where Mega Venusaur absolutely shines above Celebi is with its lack of weaknesses. Having only two 2x weaknesses to relatively uncommon attacking types (Psychic and Flying) is incredible. In comparison, Celebi's SEVEN weakesses, including a brutal 4x weakness to Bug (which is a huge problem in a world where people spam U-turn), Dark (making it Pursuit weak), and other very common attacking types (Ice/Fire) along with Ghost, Poison and Flying really hinder Celebi's viability as a Wall. Both Pokemon have an extremely useful set of Resistances, but only Mega Venusaur fully realizes the potential of this by combining it with few Weaknesses.
Ultimately Celebi shines as a Utility Pokemon, while Mega Venusaur shines as a Great Wall.
The analysis is that Celebi and Mega Venusaur both have unique advantages, and both should prove very useful in OU play on the right team.