"What is the difference if I use a 20mm wide angle lens or a lens that has a wide angle converter to give me 20mm on my otherwise longer lens?
Other than possible optical degradation, the converter is still a wide angle, correct?"
Call it anything you want, but I won't recognize a converter as a true wide angle lens. We use different terms because they're different categories of gear. I used a Sony 0.7 Wide End Converter on a V3 for awhile, and a Tiffen Mega Plus on a V1. Neither converter took filters or common lens caps. Adding the Wide End to the V3 locked the system at its shortest length, with no zooming allowed. The converters also blocked the on-camera flash, and made the camera unpocketable, since they didn't retract into the body.
Still interested in wide abngle converters?
I haven't talked about image quality yet, but that's the biggest difference. The Tiffen, which got raves on photo.net, was awful, extremely prone to flare. The Sony licked that problem, but both suffered from acute distortion of straight lines, especially at close range. They gave textbook "moustache" distortion, more like a compound bow than a simple curve.
Compared to that, the humble kit lens on my D40 is a masterpiece. There's next to no flare, better sharpness, and only a slight 1% or so of simple barrel distortion at the widest end.
So from my experience, a bottom-end wide SLR lens performs much better than a top-quality wide converter. Brand new, the Nikon lens cost about the same $150 as the Sony, but it includes a focusing mechanism, a focusing motor, an aperture mechanism, and it zooms and takes filters and lens caps. Still want to tell me that converter lenses are a good deal?