ASUS EP121 and B121 have the same exact SanDisk SSD
I was hoping to see an improvement in the SSD but the B121 is really the EP121 with a different Wireless/Bluetooth card and the TPM Chip. Other than that they look to be the same hardware.
SANDISK 64GB SSD P4
I have upgraded to an Intel 310 80GB which has increased the performance drastically. You can buy here. A decent upgrade, but may not be worth the cost.
SANDISK 64GB SSD
Intel 310 80GB SSD
Windows Experience Index increased from a 5.9 to a 7.6