At Lvl 10 it's simply whatever you feel like, though I'd Suggest Nydus Network, it makes your creep spread slightly further, and whenver you BA/AA, you'll reduce cooldowns of abilities, and the regenerating properties are amazing, and of course, you can traverse the map.
As Poke Zagara, I barely ever interact in teamfights directly, I mostly go around and poke the enemy heroes and towers/walls with my Q. Making this talent incredibly strong. Also, the Nydus Worm have a big area, and can block entire pathways alone.

At Lvl 20 I'd suggest to upgrade whichever ultimate you took.

At Lvl 10 take whichever ultimate you prefer, Devouring Maw is good for most teamfights, though it can mess up stuff with your allies ultimates if not co-ordinated, though it is usable on unstoppable enemies, and is great to zone out enemies as well, as the devouring maw is fairly big AND will block minions and heroes.
Nydus Network is also amazing though, especially with the fact that hitting enemy heroes will reduce cooldowns on basic abilities, and your mutalisk and roachlings will do a lot of damage with the 1.75% damage talent at 16.

At Lvl 20 upgrade ultimate, Fury of the storm looks interesting, but the strong part of it, the AoE effect, does NOT damage enemy heroes, not even structure.