Browser profiles

Easiest way would be to set up two user accounts and enable fast-user switching.

 

only other idea i would have is use one browser for a certain profile and a different one for your alternate. Other than this, FUS is basically your only other option

 

This is sort of a guess, because I don't really know how Safari stores user settings and bookmarks. If it uses a relatively clean system of preferences files that you can access, then you do the switching with an AppleScript or even a BASH shell script. I'm certainly not the one to ask about specifics on how to do it, but it's probably worth looking into.

The thinking is that you could have an AppleScript that when run asks which profile you want to use, then depending on what you select, renames a preferences file so that it's "invisible" to Safari, and renames another one that you want used so that Safari will read it and use it upon startup. If you select another profile, it does the same sort of thing only with another preferences file ending up with the "correct" name. Rather than renaming, you could also move the files into and out of the correct folder, but I think the renaming option is probably cleaner.