I'm pretty sure this topic has been beat to death but why not give it an extra kick. If you sit down and actually compare the two instruments what is going to be more challenging to person of average stature. Bass is much bigger than guitar, but guitarists have to hold down more than one string. Of course if you tried to play a six string bass like a guitar, it would probably be much more challenging. If you try to play "suck my kiss" by the Chili peppers on bass and then on guitar it will probably be more challenging on guitar. I think the circumstances under which the instrument is being played has alot to do with the degree of difficulty.