Assuming your purchase is an affordable one for you, and it's only a matter of which method is preferred, I'd always go credit card. Not only do you get the points/ cash back, but there are insurances on most good credit cards that you don't get paying cash or debit.
For example, my credit card offers protection should any item charged in full become lost, stolen or damaged within a certain time frame. So if your new handbag that you just paid cash for is stolen 15 minutes after you leave the store, too bad for you. Bought with a credit card? No big deal-- get the charge reversed and go buy a new one.