Anything HEPA rated, people would appreciate it when you tell them that you are not going to dust up the air with a conventional vacuum and then have it all settle on the clean carpet over the next few hours. People with allergies especially. Could just be another thing that they notice different about you.
Sanitair has some good HEPAs in the $300 range and the Proteam LineVaser is approved by the American Lung Association and should be used for things like mattress cleaning. Its $550 on ebay I saw the other day. I am a newbie so I have used neither, so take it for what is worth, but HEPA could be a nice touch.