Technically, Qt seems to have a number of advantages. Undoubtedly it has a lot more full time developers working on it. However from a governance stand point, GTK is better. It is managed by a non-profit and doesn't require anyone to sign agreements before accepting contributions. Also Qt seems to be under turmoil now after Nokia abandoned Linux development (which to be fair, also has affected GTK) and it is not clear how much of a good free software citizen Digia is going to be. If Qt would be moved to management under a non-profit foundation, that would be a great outcome.