If you're on a Mac, you can't use yWriter. :) Beyond that, from the screenshots it looks like yWriter only allows you to break into chapters, while Scrivener lets you have files in folders with no restraints on organization, as deep as you like, plus photos and audio files.
Scrivener has many tools, like Word Count Goals, frequency analysis, a name generator, a compiler, and the ability to do snapshots of your work so you can roll back to a previous draft. It's simply a more expansive, powerful program.
If you just need to get underway (and you're on a compatible platform), start with yWriter and save your money until you can get Scrivener. You can write in Notepad if you need to; you don't have to have Word.