The game's a lot more dynamic, in general, now. And it'll really depend on how you like to play which will decide which starter you want.
Because now the game has a more balanced (in general) spread of typing pros/cons, more balanced stat balances, and the attack/defense stats are split into Special/Regular, with each type having attacks in both areas.
Generally speaking, your fire types are going to end up more like glass cannons, and they're going to go down even faster if they don't have a stat-alottment advantage (as in, they are focusing on special attacks and have low defense, but their opponent is high in special defense and average in attack).
Your grass types tend to be more of a support/defensive type. Bulbasaur is going to debuff your enemy and cause general mayhem. And as you increase the number of pokemon (because the games now have 2/3x2/3-pokemon battles), it's going to get much stronger. Plus, because they're defensive, they tend to be under-prioritized. But that could be horrible for the opponent, because if my Bulbasaur inflicts both Leach Seed and a Bad Poison effect, the damage of both of those effects is going to increase each turn (and heal whatever pokemon is in the space Bulbasaur occupied), making it a really effective way to support myself in a situation where just one pokemon isn't going to cut it.
Fire types, as glass cannons, tend to get far weaker in situations where they aren't the faster pokemon, or when there are more pokemon on the field. If the other team can toss a few moves its way in one turn, it's really going to struggle to stay alive in general. And because so much of its effectiveness is built into its need for successful attacks, debilitating attacks (like ones that cause Blindness) work really well against it.
Sure, if all I'm doing is taking out random pokemon in grass, a fire type is going to get me through it faster in general. But a grass pokemon will get me through trainer battles and gyms without nearly as much over-leveling required, plus fewer trips back to the pokemon center.
Plus, because move sets tend to change, and because it's very rare for early pokemon to be something you want to keep in your party for any extended period of time, the fact that grass types are more common early on doesn't matter as much. Because it's not until you start encountering Fire pokemon that you start encountering Grass types you'd actually want to train.