When comparing Qt vs Agar, the Slant community recommends Qt for most people. In the question“What are the best cross-platform GUI toolkits?” Qt is ranked 2nd while Agar is ranked 5th.
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 Easy to work with
Pro Libraries
Libraries included for mathematics, networking, vector graphics etc.
There is also a beta library for audio.