When comparing Diagrams.net vs UXPin, the Slant community recommends Diagrams.net for most people. In the question“What are the best mockup and wireframing tools for websites?” Diagrams.net is ranked 6th while UXPin is ranked 31st.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Rich functionality for a web-based application
Pro Integrates with multiple cloud storage services
Draw.io offers saving files directly to services such as Google Drive, OneDrive or Dropbox along with saving to to computer.
Pro Real-time collaboration
Pro Great flexibility customizing text labels
Pro Completely free
Pro There is also a Desktop version

Pro Good list of different output formats
Draw.io supports export for PNG, GIF, JPEG, PDF, SVG, HTML, and XML.
Pro Internet Explorer 6-8 and touch device support
Draw.io works with every browser (even IE 6) without plugins.
Pro UXPin has great advanced features for prototyping
integration if this then dop that and custom javascript functionality
Pro Actively developed
UXPin is constantly being updated with new features.
Pro Import from PhotoShop and Sketch
A pro UXPin account will allow you to import existing projects from PhotoShop or Sketch.
Pro Extensive library of elements and patterns
UXPin offers elements from a wide range libraries. This includes responsive frameworks such as Bootstrap and Foundation, and libraries specific to mobile prototyping.
Cons
Con Import from Visio .vsd is useless
Even simple diagrams get all scattered.
Con Behaves strangely for certain diagram types
For example the snapping function for arrows doesn't work properly for the sequence diagram template.
Con Expensive subscription model
$29 per user/mo ($350 pa) will soon rack up compared to a standalone product like Axure PRO ($450) which has a license that lasts years (they have given out free updates for the last 2 versions).
Con Steep learning curve
UXPin is one of the more complex prototyping tools. Combined with a lack of documentation, it can be discouraging for first time users.
