The obvious things are that Torterra is better on the physical side while Venusaur is better on the special side (both attacking and defending) based on stats. Torterra also has a lot less speed, which means it is a better choice to use on a Trick Room team. However, outside of trick room, Venusaur has the potential to outspeed more things and take them out or setup first.
Both grass types are dual typed (and the only dual typed grass starters so far). Venusaur is half poison, while Torterra is half ground. Venusaur gains an immunity to the poison status (but remains neutral on attacks), gains a resistance to fighting, becomes 4x to grass attacks, and loses a weakness to bug and poison attacks (instead becoming neutral). In addition, due to the secondary typing, Venusaur can also use Black Sludge instead of leftovers to heal, freeing up that extra leftovers for another teammate. However, Venusaur also loses the ground resist and gains a weakness to psychic attacks. Overall, not too bad.
Torterra gains an immunity to electric attacks (upgrade from 2x resist), which protects it from electric based paralysis (outside of static). Torterra also gains a rock resist (which is perfect considering stealth rock) and also becomes neutral to poison attacks. The secondary ground typing is also noteworthy because this means that Torterra can also be used in the sand, something that Venusaur cannot do. In return, Torterra loses its resistance against water attacks and gains a 4x weakness to ice attacks. This is extremely troublesome as this means that Torterra can no longer face off against a water type as it doesn't resist water attacks anymore, and almost every water type has an ice attack in their moveset. Where Venusaur has a chance to survive a non-STAB ice attack, Torterra does not, and will go down as quickly as the psuedo-dragons.
Both Venusaur and Torterra get leech seed. Torterra (if transferred from 4th gen) gets Stealth Rock, while Venusaur gets Sleep powder and Poison powder. It is here where Venusaur shows an advantage. Besides the additional useful status attacks, Venusaur can also remove toxic spikes on your side of the field just for switching in.
It is on the physical side that Torterra shows its strength. Having access to STAB Wood Hammer and STAB Earthquake, along with Stone Edge is nothing to joke about. Torterra will put a hurt in anything that doesn't resist. These three moves alone provide great coverage and great power. Torterra also gets Superpower for extra coverage. If that wasn't enough, Torterra can either be used in Trick Room or use Rock Polish itself to take care of the speed problem before bringing the pain. Venusaur meanwhile has to struggle to come up with something noteworthy to use. Power Whip is great. Earthquake is as well, but it doesn't get STAB on it unlike Torterra. Outrage is really only great on a dragon that gets STAB.
On the other hand, Venusaur does a bit better on the special side, getting access to STAB Sludge Bomb and Leaf Storm (grass knot/energy ball). Venusaur can also make use of a hidden power if you need to provide extra coverage. Torterra meanwhile, really has no business using a special set, as it doesn't have the movepool or stats for it (although it does get STAB earth power).
Depends on what you need for your team. Venusaur is better for status, has less crippling weaknesses, and is better as a bulky special attacker. Torterra is better on a Trick Room team, as a bulky physical attacker, or on a sandstorm team. Basically, Torterra fares better on specialized/themed teams, while non DW Venusaur is better on more neutral/all around teams.
Remember that if ths is for in-game team, anyone can be a good choice, so it really depends on what you need, physical or special attacker.