I always go with the fire starter pokemon, offers a bit more challenge early on in the game, plus I think they always look the coolest in their final form, mega evolutions not included.
Grass Typing is one of the worst in the game. It has few advantages and enough weaknesses that everyone usually complains that Grass doesn't get enough love.
Just to add a little more data:
Grass deals x2 Damage to Ground, Rock, and Water, while Resisting Electric, Grass, Ground and Water.
Grass deals x0.5 Damage to Bug, Dragon, Fire, Flying, Grass, Poison, and Steel, while taking Double Damage from Bug, Fire, Flying, Ice, and Poison.
I've always thought Grass got shafted the most considering it's a starter typing.