If it's something you want to make a site about then I think you'll have to have it on the IDN domain 'exact term' with the ñ character, that way the search engines should index and find it and show it to searchers looking for that term. However, for those lazy people or the ones using international settings and keyboards, not localized ones, then it would make sense to have the non-IDN domains and redirect for the type in traffic. It won't matter if there's less searches because the site will be on the searched term domain anyway. Tbh I have no idea how to get the ñ character on my UK English keyboard btw, I copied 'n pasted from your post so I think you need to cover all your bases. Do Spanish/South American keyboards have a seperate key for that character?