Toaster's smaller and easier to clean. Quicker to start, if only slightly. Probably faster. Wouldn't be surprised if they're more power efficient per slice of finished toast. Toaster oven, being large and having so much functionality overlap with a normal stove (and microwave, to some extent), is far less common (at least in the US) than the smaller and more focused toaster. Most two-slice toasters you can easily stuff in a drawer or cabinet. Toaster ovens demand permanent counter space.