When comparing OmniGraffle vs PlantUml, the Slant community recommends OmniGraffle for most people. In the question“What are the best flowchart / diagram softwares?” OmniGraffle is ranked 18th while PlantUml is ranked 21st.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Great for web layouts
Pro Many, many integrations
For example from Emacs, Sublime Text and so on. You can learn about integrations here.
Pro Easy create diagrams from text
There is even a Visual Basic Macro to draw UML diagrams in Word.
Pro Free/libre/open source (GPL)
Source code can be browsed here.
Pro Has online editors
Such as.
Pro sphinx-contrib extension for embedding into docs
sphinx-contrib extension on PyPi
Allows you to keep your UML definitions under VCS and include them inside generated documentation with ease.
Cons
Con Help section could use improvements
Con Mac Only
OmniGraffle is a great tool, but its utility is limited by being trapped on one platform.
Con The "plantUML-language" is a bit strange and hard to grasp for new people
Con Slow development due to it being mostly a one-man project
There’s been no recent contribution to the source forge project but from the administrator arnaud_roques. Future contribution might be difficult due to lacking code documentation.