The banker, because he'll send you an email to say thanks.
Then you'll have direct contact to someone who "owes you one." And this isn't just any contact, it's a well-connected banker.
If played right you'll get far more than $20 ROI from the banker than the homeless person.
The homeless person charity case might make you feel better in the short term, but the banker will give you something better than feelings in the long term.
Either way, giving away $20 in this context would likely be for selfish reasons regardless; Donating money makes us feel good about ourselves, and it doesn't matter so much where the money actually goes.