As usual, start out with the samesies (ratings from draft.bestiaire.org):
Vassal Soul is rated #110 overall (#8 among commons); Sunspire Griffin is almost right behind it at #115 (#13 among commons). They're both 2-power, three-drop flyers, with creature types that, in draft, don't matter.
Vassal Soul comes in at a mana cost that is functionally a little bit lower, because those hybrid mana fit into a few more decks, and fit perfectly into an Azorius deck. But it also has the flexibility to fit decently well into Izzet- or Selesnya-heavy decks, especially if you find you can reasonably splash white or blue (respectively). Sunspire, meanwhile, really can't slide into as many decks, and in a draft scenario (where you're always going to play more than one color), double-white is a pretty tall order; you'll just as often find yourself able to play it on turn three as turn seven, or later.
The other advantage Vassal Soul has is with regards to removal. Obviously both cards are susceptible to Annihilating Fire or Launch Party. However, Sunspire Griffin can be eliminated by Ultimate Price (though that's probably an unwise use of such a strong kill spell). Additionally, Sunspire Griffin survives Stab Wound (our pick for the best removal spell in RtR). This means that when stabbed, you have an 0/1 flyer who deals 2 damage to you every turn. It's completely negated outside of chump-blocking, and it becomes a considerable liability if you can't get rid of it.
If we were talking about the two cards in general, I might lean towards Sunspire Griffin with its extra toughness. But in a Return to Ravnica draft format, I think Vassal Soul's flexibility and its ability to be slightly less of a liability with RtR removal puts it a tick above Griffin in this set.
Joe's pick: Vassal Soul