I agree on scintilla being better.
I don't know if this is because of gtksourceview or not, but loading large files or files with long lines freezes up gedit2/pluma, which uses gtksourceview. I used to prefer gtksourceview, but I think I prefer scintilla more now.
I could have just lacked experience, but navigating the code to add background color or select regions didn't seem as easy with gtksourceview. Their iterators, marks, and other anchor/location-types seemed to be unintuitively mixed and would become stale data quickly without even the text changing.
Thanks,
Steve