LOL! I deal with people getting hyper-excited about this subject just about every waking hour. And it's not even my staple fodder!
Peter, 35mm will do fine for star trails, and the RB67 will give a much bigger image with more detail; the Speed, ditto. You are spoilt for choices. I'm sure it will occur to you with two tripods and two cameras of different formats to go for it with both of them and come away beaming! And camping — that's the best way to go. Don't be too concerned about overly theoretical distractions like reciprocity failure. Short exposures of static stars are fine; let the film literally go feral over several hours while you pursue other things around camp. I hope you have fun and look forward to learning how you go. After all, Phoenix AZ (in some ways I think it is similar to the parched outback of Australia) is on my itinerary in 2015 (whipping tour of the heavy-hitter national parks).