Amd HD5850, going to change

i need some tips..i have a poor HD5850, i'm thinking to upgrade my gpu config but i don't know if it's better to buy another 5850 for crossfire for only 50€ or if would be better one new R9 270.. i need only to decrypt some hashes for educationl purpose..

my pc is:

CPU: i7-4770k
MOBO: bigabyte z87m-hd3
RAM: 8gb pc3-10700