I have both, though the toaster oven is a jazzed up fancy convection job. The toaster gets used every. single. day. and the oven 4-5 times a week because short of a whole turkey, there's not much I can do in the big oven I can't do there, and it uses less power, is faster, and heats up the house less than the oven, and can be moved outdoors in summer.
I will keep the big oven, though, because I do cook whole turkeys, bake in big batches, make multiple pizzas when we have guests, etc. (Plus it's attached to my stovetop.)