The difference between suet and tallow?
If you're making tortillas and pastry dough, you'd want to use the rendered suet = tallow.

If you're making something like a traditional English Christmas pudding or a spotted dick, you'd want to use the un-rendered (but cleaned and shredded) suet.

Either one would last pretty much indefinitely in the freezer. Suet would have a shorter lifespan at room temperature (since it's still got bits of meat attached).