Fastpitch bats are narrow and streamlined, with little variance between barrel and neck. Slowpitch bats have a larger barrel, which increases the sweet spot and the likelihood of making contact.
Fastpitch bats are light to allow for quicker reaction time. Slowpitch bats are heavier and take more effort to swing, but react more powerfully than fastpitch bats.
The low-end price on both bath types is about the same, but high-end fastpitch bats can cost considerably more than slowpitch bats because they use advanced technology and materials to maximize weight-to-performance ratios.
There is no discernible difference in length between fastpitch and slowpitch bats. Bats of the same length will vary considerably in weight, with fastpitch bats being lighter. Sometimes batters prefer shorter slowpitch bats to allow more control over the barrel of the bat.
Fastpitch softball bats are designed to give maximum swing speed and streamline control to the batter. Performance is an added bonus but making contact is the first priority. Slowpitch bats are designed to give maximum performance upon contact. Their heavier barrel may be more difficult to control but the same reaction speed is not necessary in the slowpitch game.