Here is an FAQ that discusses some of the differences: https://ttlc.intuit.com/questions/2942039-what-s-the-difference-between-turbotax-online-and-the-turb...
I personally favor the download product because it allows me to look at and enter data directly onto forms. You can only see the form 1040 using the online product unless you pay for the product and then create a pdf. The download product also has a "what if" feature that allows you to play with different scenarios without affecting the tax return that you are preparing. For example, if you wanted to look at filing married filing separately vs. married filing joint, you can do that with the "what if" feature in the download. It is not available with the online product.
