There are no essential differences between how the Desktop and Online versions handle taxes, but one benefit of the Desktop is that you can view your forms along the way instead of at the end when you pay. You can also handle certain circumstances more easily in Desktop.
Form 3115 is not available through TurboTax. You will either need a tax professional or do it by hand.
Below are the IRS links related to the change in accounting method. TurboTax does not have that form.
Maybe these will help.
Form 3115, Application for Change in Accounting Method
Change-in-Accounting-Method
Instructions for Form 3115
Instructions
Form 3115,
Form 3115
