There are similarities in that both are on a Ford 550, both are about the same size. The big differences are in the construction - ER = composite, Siberian = Aluminum and floor plan - Siberian is more open and has dry bath. Both are very nice vehicles. I just had a visit from a stretch Earthroamer and have just returned from a visit to Columbia, so the similarities and differences are very fresh in my mind. And both are too big for my use.
This is the beast that I examined, even the same color. It is number 125 of November, 2012.
Rather interesting set up - two photo editing stations in the rear seats, extensive spaces for camera bags and tripods and half of the upper cabinets on the passenger side is taken up with RAID storage. (The owners are photographers who are full timing in the beast.)