Forget paper or plastic -- these days grocery stores ask you if you have a discount card.

The stores say the card saves you money, but is that true? NBC 10's Consumer Alert team went undercover to find out.

Grocery cards are popular with customers.

"I use the store card because all the sale items are right on the card," said one shopper.

"I use it because I save money. I can see myself saving money," said another shopper.

But do you always save money when you use the card?

The Consumer Alert team decided to do some shopping to see what would happen.

We went undercover and compared prices at six area supermarkets -- three that have a discount card, and with three others that don't offer a card.

We put together a shopping list of 45 everyday products. Almost all of them were brand names, except for some produce and diary products.

Bread, eggs and milk for the next storm that "Hurricane" forecasts ... breakfast, lunch and dinner items ... and let's not forget the dog or the cat and the paper goods and household products.

Then we went shopping. Boy did we go shopping.

We shopped at two stores each in Pennsylvania, Delaware and New Jersey.

Our first stop was in Montgomery County where we went to a Clemens store that has a discount card and Redners store that does not offer one.

Our bill at Clemens totaled $110.92. Our receipt said we saved a total of $10.72 on 11 items with our discount card.

But our bill at Redners without the card was $92.59 -- $18.33 cheaper.

Next stop was in Delaware, where we went to a Genuardi's with a card, and a Parker's Thriftway that doesn't have a card.

Once again, the nondiscount store was cheaper. At Genuardi's, our bill came to $121.54. The receipt said we saved $7.64 on seven different items.

Our 45 items at Parker's Thriftway without the card cost $116.53 -- just over $5 cheaper.

Our final stop was Camden County. We went to the Crystal Lake Thriftway without a card and a ShopRite that has a card.

This time, the store offering the card was cheaper, but just barely.

Our bill at the ShopRite was $108.91 -- the receipt said we saved $1.10 on three items. At the Thriftway it was $110.10 -- so this time the card saved us $1.19.

Now, it is important to note that our shopping comparison was just a one week snapshot, and that stores have different items on discount each week.

Here's what the stores said in response to our findings:

Genuardi's said that its card delivers many benefits, including double coupons, saving certificates for future orders, contributions to non-profit groups and entry into sweepstakes and contests.

ShopRite said "on everyday prices we will have lower prices than our traditional competitors."

We tried to contact Clemens repeatedly, but they did not return our phone calls.

There's one other cost to consider when you use a discount card -- the price of your privacy.

If you use a card, the store will have a record of everything you've ever bought there -- from junk food to birth control to over-the-counter medications. Each of the stores tell us that they never sell the information to other groups like insurance companies.

The stores say they use it only to track buying habits and design promotions for their shoppers.

The Consumer Alert bottom line is to remember that you are giving up some personal information when you used the card; and don't rely on your receipt to determine whether you're really saving money on your shopping bill by using a card.

The best way to save is still the oldest -- shop around and compare.