If Dad already has cable TV, definitely consider the provider's cable internet service. That makes exactly the same kind of sense as considering DSL from his phone provider, inasmuch as either provider will give you a deal for "bundling" two or more services.

If his does not have cable (i.e., if he has satellite dish TV or simple broadcast only), DSL may be the only practical choice, and the current phone provider will probably offer a deal that no one else can match.

I've had both DSL and cable, and don't find a great deal of difference. Cable may be a bit more consistent speed-wise. Either is, of course, a huge improvement over dial-up, especially since neither one ties up your regular phone service.