The difference between the two is simple. Online is ONE return at a time, whereas the download is for up to 20 returns that can be completed.  Since it's only you, I would suggest the online version unless you have more than one return to do.

Here is the link to Self-Employment online: https://turbotax.intuit.ca/personal-tax-software/self-employed-online.jsp

And this one is the download link if you choose that one (click Home & Business): https://turbotax.intuit.ca/product/download/index.jsp