When comparing Diagrams.net vs FluidUI, 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 FluidUI is ranked 12th.
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 User friendly
The learning curve is relatively low compared to other software that supports animations/transitions. Both building the wireframes and designing the interactions is all drop and drag.
Pro Tools for team collaboration
Work with your team on a design. Connect on live video calls and chat or add comments.
Pro Supports realistic gestures & transitions
You can simulate all the typical iOS gestures such as tap, double tap, swipe & long hold. These gestures then can trigger animated transitions such as sliding, fading, pop in, & flip. This enables you to create very realistic interactable demos of your app. Checkout the demo page for some examples.
Pro Awesome version control
Every action you or your team takes automatically creates a revision you can easily revert to.
Pro Provides UI assets from all the versions of iOS
The UI library is huge and very high quality. All the UI required to design iO7 apps is included as well as iOS 6 and basic wireframing elements. For example in iOS7 it provides:
- Backgrounds
- Navigation & status bars
- Layouts (list pickers, maps, keyboards, scroll etc)
- Controls
- Typography
- Icons
Pro Fast
Using the app is feels very snappy.
Pro Free forever for a single project & 10 screens
They have a very reasonable free tier with the paid versions starting at $12/month.
Pro Lets you test on an iPhone as well as in the browser
You can preview your designs live in the browser as well as send them to a mobile device to better simulate the UX.
Pro Makes it easy to plan out the app workflow
Fluid isn't just about wireframing static screens. You can plan out all the user flows and then define the transitions between them. This is really handy for planning out the entire user workflow.
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.
