User blog comment:L'Ange Noir/Thor: The Dark World Review -there will be spoilers/@comment-11297966-20131111161213/@comment-24205467-20131112210150

Selvig was under the Tesseract's control for longer, and did more damage than Hawkeye (being the one who built the machine for the portal), so I think that's why he went insane. Plus I'd assume Clint has psychological training, since he's a soldier in the field, he'd need to be 'immune', so to speak, to traumatising circumstances.