I went through that and got a little bit of information, but it doesnt do a good job of comparing the Chromoly to the Reynolds, but ill post the impression I got of these metals, please correct me if im wrong.

Chromoly 4130 - Newish, most popular frame, pretty dense and heavier than Reynolds 631... No noticeable flex in the frame under normal conditions.
Reynolds 631 Steel - upgrade from the older more common 530 steel, Lighter that Chromoly, low density, a little bit of "liveliness" in the frame, Air hardened (which if i remember correctly from shop classes way back when, allows the frame to flex more. aka Reduces stiffness)
Reynolds 530 Steel - Popular a few years ago, pretty similar to Chromoly 4130
Alluminum - Lighter, less dense, less strength, more flex, cheaper, etc

Does this sound about right? and to put this in perspective Im looking at a Kilo TT (reynolds 5321/Chromoly 4130) and a IRO Mark V (reynolds 631)... I know the frameset for the Iro ways 4 lbs, how much in comparison would the Kilo be?

Also does weather take its toll on either of these materials more than the other, I live in Chicago so this frame is going to take quite a beating of rain, heat, cold, ice, snow, sleet, salt, humidity, etc.