Shaymin (Land Forme)'s BST is 600 and its stats are 100 for all 6 stats. It's other form (Sky Forme) has the same BST as the other Forme, but it receives +03 in Attack, -25 in both Defense and Special Defense, +20 in Special Attack, and +27 in Speed.
Venusaur's BST is 525 and its stats are 080 in both HP and Speed, 082 in Attaxk, 083 in Defense, and 100 in both Special Attack and Special Defense.
Shaymin (Land Forme) can learn Normal- and Grass-type moves through level-up. But, its other Forme can also learn Flying-type moves through level-up. Both Formes can learn Normal-, Grass-, Psychic-, and Fairy-type moves as TMs.
Venusaur can learn Normal- and Grass-type moves through level-up. It can learn Normal-, Grass-, Poison-, and Ground-type moves as TMs. It can also learn Normal-, Grass-, and Poison-type moves as egg moves.
Shaymin (Land Forme) is a pure-Grass-type: making it resistant to Water, Electric, Grass, and Ground; and weak to Fire, Ice, Poison, Flying, and Bug.
Its other Sky Forme is a Grass/Flying-type: making it immune to Ground; resistant to Water(1/2), Grass(1/4), and Fighting(1/2); and weak to Fire(2x), Ice(4x), Poison(2x), Flying(2x), and Rock(2x).
Venusaur is a Grass/Poison-type: making it resistant to Water(1/2), Electric(1/2), Grass(1/4), Fighting(1/2), and Fairy(1/2); and weak to Fire, Ice, Flying, and Psychic.
By stats, Shaymin(both Formes) has a higher BST(600) than Venusaur's 525 BST. Shaymin has a higher HP stat(100 for both Formes), Attack stat(103 for Sky Forme), Defense stat(100 for Land Forme), Special Attack stat(120 for Sky Forme), and Speed stat(127 for Sky Forme) compared to Venusaur's (HP and Speed of 080, Attack of 082, Defense of 083, and Special Attack of 100). Both Venusaur and Shaymin (Land Forme) have the same Special Defense stat of 100.
By moves, Shaymin (Sky Forme) can learn 3 different type of moves through level-up compared to its other Forme(2) and Venusaur(2). Both Shaymin(both Formes) and Venusaur can learn 4 different types of moves as TMs. Unfortunately, with Shaymin being a Mythical (maybe Legendary, still confused as to which), it does not learn any egg moves, but Venusaur can learn 3 different types of moves as egg moves.
By typing, Shaymin (Sky Forme) has 1 more immunity than its other Forme and Venusaur. Venusaur has 5 resistances compared to Shaymin (Sky Forme)(3) and Shaymin (Land Forme)(4). Venusaur also has 4 weaknesses
compared to Shaymin (both Formes)(5).