Hi Shawn,
Yes, the W5100 handles the TCP/IP stack while on the ENC28J60 you'd have to code the stack (too) in PBP. For me, being an ethernet rookie, it was a given especially conciddering the small difference in price. One drawback is its bloody tiny.... 0.2mm legs on 0.4mm pitch.....

There's now a W5200 which is also an SPI device (unlike the W5300) but with 32k buffer and 8 simultaneous sockets. I've only looked at the datasheet brifely but it looks very simmilar to the W5100 commandwise.

Looking forward to hear about your adventures!

/Henrik.