Nova is one of Mengsk's underlings who comes at you out of the blue and basically says, "These dudes are a huge pain in our side, help us take care of them, oh thorn in our sides." Helping her over the dude who's helping you by screwing over said dude feels like a bastard thing to do.
Either one you choose the other was lieing to you, however Tosh is closer to Raynor's moral compass in my opion, Nova has boobs.
Actually, after that mission briefing, I think they're both telling the truth. Tosh did withhold certain details, of course, but he was never blatantly lying, since Raynor never asked exactly what he was doing.
But if you pick Tosh, he stays on the Hyperion and ends up being something of a spiritual mentor for Raynor. Nova just bails like she said she would.