If you like playing sorc more than wiz and want the wiz past life then it is best to start with wiz as the first life is the shortest. However, I think the wiz past life isn't that important for a nuking sorc. The spell pen often doesn't apply to damage spells and sorcs generally don't have the feats to take the past life feat for +1 to DC (which admittedly can be useful but there are many spells which don't have a save so there is a workaround).
What it boils down to is which class will be more fun for you? If you liked sorc more than wiz before I dont think the game has changed enough to change that for you.