Hi, the main difference is in the price. Both softwares are nice for several tasks. Image drapping is not exactly celar for me. If your main purpose is to georeference images, my suggestion is the Erdas, whilst for analysis I prefer Idrisi.
Besides, QGIS (www.qgis.org) as a free platform is able to georefernce in an easy way, and with Sextante plugin (Orfeo toolbox) You can analyse them, too.
@Sebatian: You got it right! That is exactly the question. Please any headway? I will be glad if you can prove documents word or pdfs on this. Many thanks in anticipation. @Others thanks for the wonderful suggestions.