As an installer and refinisher of all kinds of hardwood floors I can honestly say that I sand thinner planks due to warping about ten times as often as wider planks. I think the opposite. Another reason my experience may be that thinner planks warp more often could be that the majority of 5 inch planks are now engineered wood which is made to resist moisture damage, but most of the thinner planks are solid wood which must be treated and installed with the utmost care to avoid moisture and thus warping. I think that the installation method used, for whichever width of wood that you choose, is going to be the main factor to decide which is better for your particular home. Either one should be able to be installed properly to resist moisture problems of all kinds. I personally don't think width is a major factor when considering warping.

I agree with Christopher... only one thing makes flooring "warp"... excessive moisture. Although poor product choice &/or installation can be factors as well.