Compare with standard size card, clamshell card is made of ABS material, which is stronger. Besides of that, the antenna of clamshell card is bigger then standard size card, so it has better reading range.
EM4200 card is upgraded from EM4100 (EM4100 card is not produced anymore). It has 128 bits UID number. This number is fixed and can not be changed. So EM4200 is called 125Khz read-only card. It is widely used in access control.