When comparing GTK+ vs Sciter, the Slant community recommends GTK+ for most people. In the question“What are the best cross-platform GUI toolkits?” GTK+ is ranked 8th while Sciter is ranked 14th. The most important reason people chose GTK+ is:
Ranked in these QuestionsQuestion Ranking
Pro Language bindings
Pro Nicer user interface when using PyGi
Creating cross platform enterprise apps with PyGi and with the help of Gtk Themes makes them look great.
Con Terrible Integration
Looks and feels like an alien on all other platforms than GNOME.
Con Linux version is not very mature
The Linux version is missing HTML/CSS features when compared to the Windows version.
Con Not fully HTML5 compliant
Lacking HTML5 functionality and W3C standards: grabbing a library like JQuery or Bootstrap and use it in Sciter will not work.
Con Not WYSIWYG
Not WYSIWYG like WebForms or WPF.