Comparing the $99 version to the B ($35):

  - Double the clock speed (1.46GHz vs. 700MHz)
  - Double the RAM (1GB vs 512MB)
  - 8GB of on-board flash
  - Gigabit ethernet instead of 10/100
  - A USB 3.0 port
  - A SATA II port
  - A PCIe 2.0 lane
  - JTAG
  - x86 architecture

Overall, probably worth the extra cost if you need the power and features. The question is, who does? I'm considering this for no other reason than I want a board in this form factor and power class that has SATA and PCIe.