Unfortunately your links only include very few information. For example, there is no clear diagram with connector pinouts.
The 5100 board includes a 5V-3V3 regulator, but can it be used with a 3V3 supply as well ?
Nice to see you added a "useful links" page, but maybe you should populate it with something (like a link back to here 
Could you also make a check on the consumption of the boards as said in the 28J60 review ? Regarding this review, I'm surprised that michael had DHCP issues, for me (using a DINo) it worked at the first try.
@All, I relaised that this post does not get high on search because some keywords aren't present by accident. For example Ethernet Ethernet Ethernet !
I would also like to add some nuances to the other posts in this thread. While the 28J60 indeed does require a bigger load on the CPU (no TCP in HW for example), this could also have advantages : for example, it is possible to support 6lowPAN in the future 
Also, DHCP is supported standard in EtherCard library, it's just a matter of choosing static or not.