As far as I know, what you mention about "Belly of the Beast" is incorrect. This mission does NOT require Tosh. Thus, you can feel free to pick Tosh or Nova without affecting any of your later mission choices. Tosh does not participate in Belly of the Beast even if you do have him. The mission consists of Raynor, Tychus, Swann, and Stettman only.
That said, there is a strategy that can be found here that relies on using ghosts specifically for the final mission, All-In. That strategy can be found here. Thus, if you want to try that, you'd need to choose Nova.
Other than that, your choice will not affect any future choices in the campaign, except for of course which flavor of cloaked ground unit you prefer most, in the event you actually even make spectres/ghosts at all. The two abilities that differ are: Snipe (ignores armor) for Ghosts and Psionic LashUltrasonic Pulse for Spectres (area damageAoE stun attack).