Which database to choose between MS SQLSERVER or PostgreSQl [on hold]

I am developing re-engineering a system so that all my existing desktop system can be hosted in a single server & single database (a sort of SAAS model).

While combining all the database the new database will be 500+GB. More, the expected concurrent users will be 5000+ users.

I am confused on which databased to choose between SQLSERVER & PROGRESQL.

Thanks Shakti