Cash exists as a fiat currency, under full governmental control. So if the government wanted, they could simply print another $10000000 whenever they wanted. With Bitcoin, you don't have this problem, as the algorithm is incredibly secure and currency can only be earned by mining (ignoring the possibility of a highly unlikely 51% attack). Also, unlike physical cash bitcoins are non-refundable, eliminating the possibility of a fraudulent refund.

OF course, with cash you have zero transaction fees and the exchange is instant. With bitcoins, transactions take quite a while if waiting for multiple confirmations and with smaller transfers the transaction fee can be bigger than the amount paid. So bitcoins are more appropriate for bigger amount of money.