My suggestion is play both and cover each side
What starts out as a simple 1/1 generator turns into a Scute Swarm factory once you hit your sixth land and beyond
Soon enough the Treasures will allow you to play more spells and get more cards to generate more exiling effects with cards like Outpost Siege, Magmatic Channeler, and Harnfel, Horn of Bounty
Niv-Mizzet has a couple of ways to make it work, but the concept is very simple
In addition to this army of massive creatures, the deck features a number of utility spells including mana rocks, numerous removal spells, ramp spells, counter effects, etc
If you look at the image, youll notice that you should mostly play 2- and 3-cost cards