After some testing, I've found that for sorcs, the lightning staff is significantly more powerful for the ultimate than the fire staff. When I did my testing, I was on my Breton, had all passives, and used sharpened gold quality staves. The lightning staff ultimate hit 10 times for 10,728 each tick on non-crits. The fire staff ultimate hit 8 times for 11,134 each tick on non-crits.
Lightning staff:
107,280 total damage w/o crits
Fire staff:
89,072 total damage w/o crits

If you were to have a DK in the group using engulfing flames, the fire staff would be at 97,979. If you also apply the dunmer bonus to fire damage, that would increase to 104,838, which is close to the lightning staff, but the lightning staff ulti would also increase to 111,571, keeping itself ahead. As a sorc, this'll also proc implosion, making it that much better. Blockade is still going to be best on a fire staff, but if you're running destro staves on both bars, it may be worth switching which staff you use this ulti on.

Disclaimer: Shooting star will still do more single target per ultimate cost on stationary enemies and will outperform everything if there are multiple enemies clumped up. It also buffs your max magic, which increases your damage done. If you slot shooting star for the extra max magic, though, and don't actually use it, you're better off slotting a sorcerer skill for the 2% extra spell damage.

edited to include point about implosion.