Let's look at an example, assuming the following stats: Magic 6, Spellcasting 6

Confusion

Force 6, Drain 3S, Max Hits: 6, Max Penalty: -6

Force 12, Drain 6P, Max Hits: 12, Max Penalty: -12

Expected Average Hits: 4, Expected Average Penalty Inflicted: -4

Agony

Force 6, Drain 1S, Max Hits: 6, Max Penalty: -2

Force 12, Drain 4P, Max Hits: 12, Max Penalty: -4 or incapacitation

Expected Average Hits: 4, Expected Average Penalty Inflicted: -1

The drain is much lower, but the effect is also much lower. If you look at Force 6 Agony, the most you can expect to do is a -2 penalty. If 4P Drain doesn't bother you, then why not just go for a 3P Drain with Confusion? Either way, the Force only limits the hits you get. With the assumed 12 dice pool, you will probably still only get 4 hits no matter what spell you are using. Confusion will net a -4 penalty while Agony will only inflict a -1 penalty.

If the target is already injured with physical damage, there is a chance you can completely incapacitate the target, but if you are looking to hinder your opponent, confusion is much more efficient. Also, if the target only has stun damage (from a brawl, or has decent armor to turn received physical damage into stun), then you are still starting out with 0 physical damage and you probably won't incapacitate the target.

To expect to get 10 hits, you need to have a dice pool of around 30. If you could get ten hits, why not just inflict a -10 penalty? On the other hand, maybe you like causing pain. If that is your goal, obviously Agony, by its very nature, is the way to go.

EDIT: And of course, Dakka Dakka gets there first