Both card readers work great and provide a secure and integrated payments experience powered by Shopify Payments. The Retail Card Reader ships with several plastic pieces for providing a secure fit on an iPad or iPhone. This card reader is more suitable for retail stores accepting higher volumes of credit cards. You can plug a micro USB cable into this card reader to continue powering the iPad.
If you're taking payments on the go or only occasionally accepting credit cards, the Pop Up Card Reader is a great and affordable option. Due to the shape of the audio-jack this card reader can pivot slightly, but the brim allows you to support it against the edge of a mobile device.
Hope this helps!