The Safari Tech Preview is a full-fledged copy of Safari that bundles its own WebKit frameworks rather than using the system frameworks. As it's a separate application it can have a different name, and use an independent copy of preferences, history, bookmarks, etc. from the system version of Safari.

WebKit nightly builds run your system version of Safari with an updated version of the WebKit frameworks. This means that the application you're running is your normal version of Safari. The WebKit nightlies do some shenanigans to convince the system to do things like display a different Dock icon, for instance, but for the most part they just let Safari be Safari.