IMHO, both types offer very poor response rates. People view them as a nuisance.
I'd suggest looking for something that targets your specific audience, rather than trying to saturate an area and hope someone is interested.
Of the two, direct mail is worse because of the waste of resources (paper, ink, etc.) and the fruitless man-hours of work preparing, transporting and delivering useless items which just end up in the waste stream 95% + of the time.
As an aside... I'm not sure what this question has to do with Android or the immediate functionality of your device?