Hi and welcome!
if you want to be able to fallback to windows you should install on the external drive. if you don't have anything important on windows, you can wipe it from yuor hard drive, so you can't fallback. not being able to fallback to windows is also good because in case you have any problems in linux you just have to solve it...
greetz,
.-=~ iluvatar ~=-.
ps: I like your signature