Here's some reasons that may or may not convince you:
The official list of new features: http://kb.gonitro.com/knowledgebase#/answers/Nitro Pro/000002288
The Nitro 11 release notes: https://www.gonitro.com/product/documentation/release-notes
Nitro 10 isn't being updated, except for any security updates it might get.
If you want to use Dropbox from inside of Nitro, you'll need Nitro 11 as Nitro 10 doesn't support the new Dropbox API.
Hope that helps!