biggers cogs spread the wear over the teeth. [i haven't explained that spectacularly well.], so your cogs will last longer [in theory]. you'll also have more teeth engaging with the chain at any one time, so you'll be less likely to throw the chain.
as far as disadvantages go, i'm not sure that flex would be an issue with a straight chainline, [anyone beg to differ?] so the only one might be weight.
personally i'd go as big as you can until you get chainstay clearence issues.
fsnl
sparky