Honestly, they are both Grass-types, but in my opinion, its their second type that seperates them.
They both have the same ability (chlorophyll) and their stats can always be boosted with the permanent buffs you buy from the Celadon Dept. Store.
However, Exeggutor is a Grass and Psychic, while Victreebel is a Poison. This means they have different available moves.
Poison is helpful, if you can land your attacks. Poison your foe, and it doesn't matter how evasive they are. That damage will keep piling up, eventually fainting them.
Psychic has its pros too, however. Moves like confusion and hypnosis can confuse and put your opponent to sleep, respectively. These both have their place, as a sleeping Pokemon is completely helpless. A confused Pokemon will either hit you, or hit itself, making it useful as well.
Poison and Psychic will also have individual type match-ups.
When attacking, Poison is effective against Grass, but is weak against Poison, Ground, Rock, Steel, and Ghost.
For Psychic, it effects Fighting and Poison, but is weak against Steel, Psychic, and Dark.
So its all about your preference and who your opponent is. I'd say take your pick, and grind them up.