Office VSTO Add-ins vs Office Add-ins using Office JS API

Recently Microsoft introduced the Office Add-ins architecture which allows for developing add-ins hosted remotely and run within IFrames inside office. I have read a lot trying to understand whether this architecture is meant as a replacement for VSTO or do they have separate use-cases. VS 2015 has templates for both.

In my specific case, I want to develop an add-in that extends Excel 2016 with custom import functionality (e.g. custom CSVs, TSV, or even XLSX). I can't tell which type of project I should go with.

Given the amount of legacy applications developed as COM and VSTO add-ins I find it hard to believe Microsoft would drop support within the next 10 years.

For me the most important differentiators of one approach vs. the other are:

COM/VSTO Office add-ins

• Access to complete object model
• Local machine interaction, e.g. file system
• Available on Windows since Office 2007
• Support more UI control including run time enable/disable Visible/Hidden

JavaScript Office Add-ins

• Portable between Windows, OS X, iOS etc. (not complete everywhere yet though)
• Easy integration with online services
• Easy distribution, no need to worry about installers
• Support only few Button and not support runtime update for Outlook

In your case I'd ask myself these questions:

• Will there be a need to support other platforms besides Windows in the future? > JavaScript API
• Are the requirements covered by the current JavaScript API implementation?