They differ in sound signature.
The HD490 pro has 2 'sound signatures' and the HD550 is kind-of in between those 2 signatures.
HD550 is intended for hifi listening.
HD 490 PRO is intended for pro use, mixing (with an emphasis on clarity) and 'producing' which is supposed to mimic how speakers in an average room could sound tonally.
Neither are intended for gaming but the 550 would probably be the best choice for that (not a gamer myself)
With EQ you can make them sound very similar so that would leave you with costs and comfort as decision point.