Having two grass Pokémon in a team is a waste of a slot.
The following is just a broad examination based on what I have seen across the interwebz. I realize that this doesn't apply for everybody, but I think it works on a majority scale. Maybe I'm being a bit unfair to Chespin. Please don't hate, but instead comment telling me what starter you're going with and why if you do not fall into the categories listed and especially speak up if you are a part of a category I listed for one starter and picking another one entirely.