I use Smartsift for almost 4 years & my cats don't have problem using it.

Regarding the smell, are you referring the smell in general or the poop still smell even after buried? I did tried Oko Plus but it was two years ago.

Pellets (pine or any kind of wood) would be the cheapest IMO. It should last longer than clay. The only downside is the wood dust but this depends on your cat behavior.
For smartsift I use standard clumping clay litter.
I have two smartsift for two group of cats. One smartsift with closed door & another one with opened door because one group know how to push the door. I put one of the smartsift in front of the bathroom & another one in the back room. I clear the plastic bag twice a day because I have many cats but I did not change the plastic bag frequently. There's a bit of sand in there that prevent the plastic bag from getting dirty easily.

BTW, after 3 years, the handle to rotate the internal litter box broken near the hole that used to lock it but I managed to buy replacement parts. Bought extra too for future.