When comparing Diagrams.net vs Justinmind, 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 Justinmind is ranked 13th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Completely free

Pro Rich functionality for a web-based application
Pro There is also a Desktop version
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 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 Real-time collaboration
Pro Reusable components
You can create reusable components that you can use over and over again.
Pro Adaptive and responsive designs
You can create adaptive and responsive designs that will react to the viewport/screen size of the user.
Pro Functional prototypes
You can create HTML prototypes with full interactions.
Pro Supports low, med, and high fidelity
You can create simple wireframes, pixel perfect screens, or anything in between.
Pro Lots of online tutorials
The free Getting Started area has plenty of advice for getting started with Justinmind.
Pro Free forever
After one month trying the pro version, it changes to a completely free version with a few limitations.
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
Pricing is $19 per month or $495 for a perpetual license.
Con Moderate learning curve
