Torterra, hands down. Here are my reasons.
-Torterra has a more unique type combo while Venasaur shares its type with Pokemon like the Bellsprout and Oddish lines (Bellossom excluded)
-While both secondary types have a synergistic nature with the primary type, Torterra's seems more balanced
-Torterra, thanks to it's Ground-type, can counter all its weaknesses with one Rock-type move, which Ground-types can learn, something that the Poison-type of Venasaur can't do
-Torterra's "origins" is far cooler than Venasaur's, being based off the turtle that carries the world and stuff. Now that's pretty cool
-Torterra can naturally learn Earthquake without outside help, while Venasaur needs the TM to learn it, thereby giving Torterra a more reliable move right away
-Torterra has a more diverse move pool. On it's own, it can learn Grass, Normal, Dark, Ground, Ghost (Curse), and Water (Withdraw) moves by level up. Venasaur is more limited, it can't even learn any damaging Poison-type moves by level up.
-Most of Venasaur's best level up moves have bad drawbacks, require some prep time, or are fairly inaccurate (Petal Dance, Double-Edge, Solar Beam, Sleep Powder, Poison Powder), while most of Torterra's moves don't have the same problem, with only two having "major" drawbacks (Wood Hammer and Leaf Storm)
-From a superficial view, Torterra looks a lot cooler than Venasaur
-Torterra fits its role better than Venasaur, since it can be both a defender and a physical hitter, while Venasaur is a bit more vulnerable in some places
Truth be told, I love them both, but Torterra is my preferred choice. I've used both of them over my time as a Trainer, but Torterra fits my needs far better than Venasaur. I like them both, but Torterra would be my choice between the two.