Growing up we had one of the later, sliding lever controls, and now I have a T20 (1949-50). I found the regulation to be better on the later, lever models. Sometimes i have to unplug the toaster early to get the desired light toasting on my early T20. That said, I'd rather pull the plug once in awhile in exchange for the nifty sunbeam design on the side, and the cloth covered cord. The knob regulator is much cleaner looking which adds to the streamlined effect. This toaster was significantly less "streamlined" than many toasters of its day, which is probably why the shape carried into the late 1990s with no visible changes.
Funny story though...I remember watching old cartoons growing up that showed toasters with levers on the sides, where the toast popped up, and thinking how interesting that was, and how antiquated it must be. When our "Automatic Beyond Belief" toaster finally bit the dust, my parents bought a cheap crappy toaster, and I mused about the quaint mechanism by which it operated. It was only as I got older, that I realized most people's toasters worked that way, and that my experience had been the exception.