Ok, I did more research and I think I found the answer. Pete's post above about isoxaben giving better broadleaf control tipped me off. Incoming info dump.
I read the labels of prodiamine, gallery df (isoxaben), and snapshot. Prodiamine is mostly only labelled to control grassy weeds. On the other hand, snapshot/preen have a mile long list of broadleaf weeds they control.
Pete linked isoxaben above. This is only one of two ingredients in snapshot. I like that it's sprayable, but it doesn't seem that Trifluralin is also available in liquid form, so if you went a spray-only route you're missing out on some coverage you'd get by using the granular.
So, the answer is: snapshot and preen control a significantly wider range of weeds with the tradeoff of not being labeled for turfgrass. You can use prodiamine or dithiopyr for convenience of not having to buy a separate product, but it won't control as many weed types.
Also of note is regular preen is just Trifluralin and claims 3 months of control. Preen Extended Control, which is the same as snapshot but more expensive per sqft app, adds isoxaben to the mix and claims 6 months of control.