My own usual uninformed opinion (I own a mondolin which I don't play--never have: it's ugly as sin and I really feel stupid about having bought it; I keep it hidden under the couch): isn't the octave mandolin lower by several frets--making it maybe a third or minor third lower? That would be a consideration--but--and here I am an expert--don't buy it without playing it along with someone elsw with whom you usually play, or perhaps a salesman at the store, so you can decide if it is different enough to be worth buying. Also, have someone else play it so you can judge the tone from in front of the instrument.

--seed