In Tesco you resgister your Clubcard with the system, then scan your card which then releases a handset.
In Asda you register your email address and phone number, then enter your phone number to release a handset.
At the moment (in our Asda) you can only pay with a card, in Tesco its cash or card.
Checkout is easier in Tesco - you scan a barcode above the till. In Asda you scan the screen then present the handset to a scanner on the machine.