I take both. Most are set up to take plastic. But if time is precious, cash is a much faster transaction. Sometimes the "process" time for credit cards is just too slow, and there is so much to see and do other than stand and wait.
The vendor pays a fee to the credit card company , so they make a bit more money if you use cash.
I will tell you to use a "fanny pack" a purse just gets to be a real pain/nusance. It gets to hard to keep track of your purse when you have other packages in your hand. You want to keep you hands free.