The decision is easy when you know your users' process. So, despite any recommendations received here, you still need to talk to the cashiers who will be using the system and observe their interactions with the current system.
In your situation, it looks like the second one is the best since its item customization workflow is the most straightforward.
Here's how a POS system should be designed for efficiency and smooth UX during the basic operations in a checkout transaction (add item, add more than 1 item of the same kind, customize order, and remove items from the bill):
Usually, the buyer already knows the products to be purchased (they're either in the shopping cart or in the head, in case the clerk/cashier needs to bring them). Therefore, when a product doesn't require customization (pre-packaged goods) there's no need for an explicit "add" button (that's how barcode-based systems work - scanning means adding to the total).
Multiple items of the same kind
The system may prompt for the quantity as the item is added but this introduces an extra step when only 1 item is needed. An elegant solution to this would be providing 2 ways to change quantity:
- a new option shown on the main screen after an item has been added called "Previous item quantity" that brings up a dialog to modify the total number of items of that kind
- the ability to tap the quantity shown in the bill section of the screen to bring up the same dialog
Customizing an order
A number of industries have a more complicated checkout process because they sell bespoke/made-to-order goods. This requires an extra step (customization) in the process. The best way to design the POS for it is to show all options on one screen grouped into their respective categories because buyers may not follow the intended customization process (you don't want cashiers switching back-and-forth between screens) with an explicit "add" button to tell the system when customization is completed.
It shouldn't be hard to remove an item when a wrong one is added or buyers change their minds. Many POSes have elaborate "loss prevention" processes in place that require multiple screens and confirmation codes. However, the easiest solution is a cross/delete button at the end of the line in the bill section of the screen followed by a quick confirmation screen (if loss prevention so desires).
Thus, the perfect line item in the bill section should look like this (elements in brackets are interactive):
Qty. Item Price Total
 Awesome cake ¤2.99 ¤5.98 [X]
There's only one part of the process left that needs to be researched on case-by-case basis: what to do after an item is added. The main options here are to return to the list of categories or stay in the last category or go to another higher-level screen.