You guys amaze me. All my bikes feel the same to me, 531, A&P kromo, plain gauge or butted, columbus, tange no 1, whatever. Even aluminum I don't think I notice a difference. I have them all set up more or less the same, but I notice differences in the brakes, gears, etc, and maybe tires, but even then I'm not sure.
The biggest difference between bikes I've owned lately was between the ~1950 Carlton Continental with a full DB 531 frame set, and my 1951 New Hudson Silver Arrow with plain gauge 531 main tubes. Both were 23" frames and had different geometry, and both had 26" steel rims. The Carlton was more nicely equipped and several pounds lighter overall, but when it came to how the frame and wheels responded to pushing hard or going over rough surfaces, the difference was very appreciable! The geometry differences certainly played a factor but I had myself positioned over the frames very similarly.
If such observations weren't grounded in some kind of actual difference, then methinks generations of riders would not have bothered paying extra for the butted tubing and the craftsmen would not have focused on building almost exclusively with the stuff. Weight and technological innovation was certainly motivating factors back in the day, but if all you could say was that your bike weighed a pound or two less for all the extra expense of having a frame made from quality tubing, I think it would not have been held to such high standards.
Come to think of it, I've also noticed rather drastic differences in stiffness between butted and plain gauge 531 tubing on two different 25.5" Raleigh Super Course frames, and both were equipped with rims laced to Weinmann concave rims and similar quality tires.
-Gregory