The newer model is thinner and wider than the older version.
The main reason for the change is to comply with new security features on new credit/debit cards. You may have received a new card in the mail from your bank or credit card company which has a SIM card. This new device provides an additional layer of security for your card.
If you have one of the old readers, do not fret. Square will provide you a new reader at no additional cost. Just click the link below for more information and to get your new reader.
In addition, Square now offers a device to accept Apple Pay payments. They are giving away 250,000 of these devices. Check it out.