Choosing a bat with the right length to weight ratio is an important decision.
High end Fastpitch bats are designed to be either "evenly balanced" or "end loaded".
Bats with a bigger length to weight ratios; -10, -11 or -12 are usually considered "evenly balanced" bats. The weight of an "evenly balanced" bat is distributed evenly through the barrel. The lighter the weight, the easier it will be to get it through the zone. Younger players will generally have more success with an "evenly balanced" bat.
Bats with smaller length to weight ratios; so lower numbers like -9 or -8, are usually considered "end loaded" bats. The extra weight is an "end loaded" bat is distributed more towards the end of the bat, making it a little heavier swinging bat.
Most players prefer a lighter swing weight, but if you are a stronger player and can handle a little more weight, an "end loaded" bat is going to be the bat for you.