I would recommend the SEM BRUSHABLE Rubberized Undercoat (Mfr #: SE40524).
While the spray version is definitely quicker and easier to apply, brushing it on (or jamming it in there) ensures a better contact with the bottom of your car.
(Some people argue that undercoating is a bad idea because it can hold in (and hide) moisture, which actually can promote rust rather than stop it. )
I'll take my chances with the double protection of paint and undercoat. It doesn't hurt to get your car on a lift every 6 months or so and take a look.
