Hello dammien,
Yes, speed. What moved me with this Ethernet interface, besides the exercise of help building a future Ethernet API for Due, was the ability of SAM3X8E's EMAC controller to work with RMII at 50MHz, thanks to the fact that all transfers between the EMAC and the core through the DMA interface are 32 bit words (FIFO 128-byte). 100 Mbit/s pure. There are of course more embedded characteristics you can find in page 1267 of the SAM3X specs (Atmel doc11057). We can implement also lwip network stack, or NUT/OS or even the venerable µC/IP (now 6LoWPAN). I don't remember now but I think W5100 only has inbuilt IP stack, but without any doubt, W5100 remains to be a phenomenal Ethernet controller. Indeed, It should work for what you want.