The differences between fastpitch and slowpitch gloves are minor. However, the minor differences can be significant if you play softball at a high level. For example, a glove used by a Division I fastpitch college player is going to be significantly different than a glove worn by someone who casually plays in a slowpitch rec league.
The biggest difference between fastpitch and slowpitch gloves involves the catching position. If you are a slowpitch catcher, you usually wear just a mask and perhaps a chest protector. Since the pitches are slow and no stealing is allowed in slowpitch, you really don't need a catcher's mitt. A first baseman's mitt or utility mitt should work just fine. A fastpitch catcher must catch pitches in the 90 mph range, dig balls out of the dirt and throw out base runners, and therefore needs a catcher's mitt with the same type of extra padding in the fingers and heel that baseball catchers use. A scoop toe design help you handle pitches in the dirt and pitchers prone to wildness.
For other position players, gloves for fastpitch differ little from gloves for slowpitch. Even though manufacturers sometimes produce separate gloves for fast and slowpitch, Best Softball Mitts says this is primarily for marketing purposes. Gloves for outfielders are bigger and have deeper pockets than gloves for infielders, since outfielders want to ensure the ball doesn't pop out of their glove. Infielders need smaller gloves with shallower pockets to allow them to get the ball out of the glove and transfer it to their throwing hand as quickly as possible.
Fastpitch softball players on high school or college teams are more likely to need the highest-quality gloves on the market. There are three grades of leather gloves: top grain, premium steer hide and full-grain leather. Top-grain gloves are the lesser of the three in terms of quality, but they are perfectly fine for slowpitch rec league players, and need little if any time to break them in. In fact, casual slowpitch rec league players might opt for a synthetic glove, which is cheaper than leather. Premium steer hide gloves are heavier, stiffer, more durable and require longer to break them in. Full-grain leather gloves are an even heavier and higher grade material.
Gloves for both fastpitch and slowpitch have an open- or closed-web design. Infielders tend to like open-web gloves, which are lighter, but some prefer a closed-web glove that allows you to shade your eyes from the sun when catching pop-ups. Fastpitch pitchers usually prefer closed-web gloves to shield their grip from the batter to avoid tipping their pitches. However, slowpitch pitchers have no need to shield their grip, so whether to use an open- or closed-web glove is up to personal preference.