We also have a soda stream, which we alternate between using constantly or being too lazy to operate. Cost-wise it doesn't seem to make much difference for us since we buy the dirt-cheap soda at Shaws, but when in sodastream mode I will drink a decent amount of seltzer as well. Husband doesn't drink seltzer, only flavored soda.
I'm on the "Eat-as-much-as-possible-while-still-losing-weight" diet. It's working.