The basic difference between these two types of pitching machine is of the way or the style by which these two throw the ball. However, both features some advantages that might dominate the other and might even decide which one wins the match.
While a wheel design consists of one or more than wheel which is used to throw the ball, an arm-action machine, as it names indicates, is made with a steel arm that hurls the ball towards the player. Generally for a wheel pitching machine, the greater the number of wheels the variety of pitches it can propel. However, an arm action has the advantage that it mimics the real throw by that of a person and also the bowl to be seen for a longer period of time for the batter to evaluate their shot but the throw of such baseball pitching machine is limited to only fastballs as they cannot hurl junk pitches.
A wheel pitching machine can also help improve a batter’s technique but for that it is essential to buy one which has a long and transparent chute so that it takes long for the ball to be propelled and the batter can also observe it. One feature that both kinds of pitching machine may or may not offer is sound. A great intensity of sound will definitely distract a player or would cause him to rely on it to get ready which serves as a disadvantage as there are no such sound in actual condition.
To be precise, all kinds of pitching machines whether wheel, arm-action or soft toss have their own pros and cons and it all depends on the player which machine they would prefer on the basis of their budget, space and expertise. Therefore, it is you who decides which type of pitching machine actually wins the competition.