It depends on your use case. My "life work" consists exclusively of things I've typed. These types of files tend to be small, and lend themselves to being written in Google Documents. If I use Emacs, then the files are tiny and I back them up to Google Drive in about 2 seconds. This costs me all of $0 and is very easy.

But maybe your life work also includes a bunch of pictures documenting your experiences. These, and other large files, will quickly exceed your 15 gigs of free storage. Then you're probably looking at an external hard drive or cloud storage. The better fit will depend on things like your internet connection, which USB standard your computer has, your tech level, how much stuff you need backed up, whether you travel a lot, whether you'll lose or damage the external hard drive, etc.

And then just use Yvain's method to find the best one.

Of course, there's more elaborate solutions for power users, but by the time you're high enough level for them, you're a power user and don't need to ask.