555 is a nice one and easily marketable. So is 50. Without obvious dollar value I would personally prefer the 555. But being a domainer and having such a choice I would make the $$.com choice being the 50. No question about it for me.
555 (given that is the number) can easily be coined and nicknamed. Fun comes to mind. when i see 50, age comes to mind, and 50 is not exactly a fun age. Ièm sure investment, retirement and insurance type services can be offered so there is money there.
Good Luck.
Jack