I generally just use my debit card out of habit because its the first card I got and that way I don't have to pay any credit card bills at the end of the month. For purchases over $400 (since that's my daily spending limit on debit) I use my Blue Amex.
For really big purchases (airplane tickets, etc.) I use my parents' United Airlines card so they can get miles.
For really big purchases (airplane tickets, etc.) I use my parents' United Airlines card so they can get miles.