The sublime-project file is a collection of folders, settings, and build commands that are specific to that project. Opening a sublime-project adds it to the list of recent projects, which is easily accessed via the menu Project > Switch Project… or by a keyboard shortcut (Command-Ctrl-P on Mac). This is great for switching between projects, while only keeping one window open (or by changing the focus of that one window).
Find out in more detail at the unofficial documentation for Projects:
The sublime-workspace is something akin to a cache file, and generally should Not be committed to SCM. This is where in-progress files are stored when "hot_exit"
is enabled. This allows Sublime Text the ability to recover work after a crash.
I would recommend you investigate Source Control Management such as git and GitHub or BitBucket. Not only will it save you in cases that you wish you could tinker without making duplicate projects, but also recover old versions or remind you why you made an esoteric change some time ago.
My Personal experience with using DropBox as a sync method is very poor. Timing differences on when one computer would sync left some in-progress files on both machines in something of a limbo. Learning git saved me both from my mistakes of making edits on a project from two locations without syncing back to the cloud, and merging the diffs from both back into a singular canonical result.