How to add products in this point-of-sale system?

I was wondering which is the best way to choose products from a desktop Point-of-Sale (POS) application using a touch screen.

Method 1

This application has categories at right top buttons and products at right bottom buttons. When you click a product it is automatically added. I don't know how preferences are added but at other applications you click a special button to add preferences.

Method 2

The other approach is to have one panel. When you click a category you go into its products. When you click a product you get into preferences if it has them. Then you have to click the tick button to add it. In my case 55% of the ordered products do have preferences. People seem to prefer the tick button (confirmation) although I can't decide whether it is good or not. This approach gives you the benefit that you can do more with the list. You can select multiple items and aply actions to them easily. In most of the software that follows the other approach, you have to switch windows to get a selectable list. (cancels, returns, receipts, payment and selective version of them)

How can I decide which of these methods is better?