I have a tiny thing to add, even though Magnolv posted an excellent review of the topic at hand.

I tour with a slim tubed 4130 chro-mo old school mountain bike frame. I love it, but with 25+ lbs slung on the rear rack, you really notice the frame's inherent flexiness - esp in the rear triangle and lateral flex at the bottom bracket. The bike is cushy and springy alright, but sometimes I wonder how much more (subjectively) responsive a alum frame would feel. So this eventually boils down to user preference, because a noodly frame and a stiff frame will both get you to the top of a long mountain pass. Some folks prefer the micro-damping springiness of steel tubes while others prefer the relative rigidity of alum. The c'dale frames come with chro-moly forks, so the front ends on both the 520 and the c'dales should transmit similar levels of shock. The difference in vertical compliance in the rear triangles of the two bikes can be overcome with wide tires, a suspension seat post, and/or a gel seat. You cannot modulate the way bottom bracket stiffness affects the "feel" of the ride, however. Like I said earlier, I love touring on my steel bike, but the amount of flex can be nauseating for some.
One last thing - you cannot understand what a bike will feel like on a loaded tour unless you actually load it (~15-25lbs is fair) for your test ride. It is a gross understatement to say that a bike's handling changes when it is loaded.

Peace
BK