Have any of you done one or both of these? They're tempting!! They've been on my Things To Do list for some time, so I keep an eye on what happens each year. I met one of the ladies who did the 350 mile distance (I believe) on the Gold Rush Randonnee in 2005 ... some of the people who do these events are long distance road cyclists, who take to the ice and snow in winter.
I think I'd want to do the Susitna first, to get a feel for the conditions before I did the 350 mile or 1100 mile distances of the Iditarod.