When it comes to looks, Omnigraffle definitely excels.
Omnigraffle for Mac comes with a 14-day trial.
Omnigraffle has better stencil options and more templates, among other things.
You can also import diagrams from Visio, Gliffy and Omnigraffle once created.
The only thing Omnigraffle does better is that it offers a flat rate;
As I mentioned, Omnigraffle does require installation and it's
only available for Mac OS X 10.10+ and iOS 8.
Omnigraffle gives you a decent amount of features in the Standard version,
but for many, it will not be enough.
Note: Even if you already have the desktop version of Omnigraffle, you still have to pay an additional fee($50-
$100) for the iOS app.
Some of these features are also available in the Adobe CS suite,
but if you do not have the CS suite, Omnigraffle is good value(~$100) for producing detailed wireframes.