Office Developer: Difference between Office Web addin vs COM/VSTO addin

COM or VSTO add-ins are earlier Office integration solutions that run only on Office for Windows. The major difference what you see is that COM addins will be running in the user device or in the Office Client. The new Office Add-ins don't involve code that runs on the user's device or in the Office client. For an Office Add-in, the host application, for example Excel, reads the add-in manifest and hooks up the add-in’s custom ribbon buttons and menu commands in the UI. When needed, it loads the add-in's JavaScript and HTML code, which executes in the context of a browser in a sandbox.

In general, Office Add-ins provides advantages over add-ins built using VBA, COM, or VSTO. I can name few of them,

• Cross-platform support:
  Office Add-ins run in Office for Windows, Mac, iOS, and Office Online. So your solution can run in Office across multiple platforms, including Office for Windows, Office Online, Office for the Mac, and Office for the iPad.

• Single sign-on (SSO):
  Office Add-ins integrate easily with users' Office 365 accounts.

• Centralized deployment and distribution:
  Admins can deploy Office Add-ins centrally across an organization.

• Easy access via AppSource:
  You can make your solution available to a broad audience by submitting it to AppSource.

• Based on standard web technology:
  You can use any library you like to build Office Add-ins

Hope this helps.