When comparing Qt vs IUP, the Slant community recommends Qt for most people. In the question“What are the best cross-platform GUI toolkits?” Qt is ranked 2nd while IUP is ranked 14th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Open-source
Pro Global community
The community behind Qt is both massive and approachable. Digia (also owners) are joined by the likes of Intel, KDAB, ICS, Canonical and numerous others in sponsoring development, while communities such as KDE also contribute significantly. Forums are active, mailing lists are open, irc channels chatting, git repositories well managed. Answers to questions are usually minutes away.
Pro Good documentation
Professional, thorough documentation with examples, available either in a web browser or in the stand-alone desktop client called assistant.
Pro Lightweight library and simple C interface
Pro Native controls on each supported OS
Pro Just UI
Easier to mix with other libraries since it doesn't try to control all that.