I have a steel frame and am now looking to build up an aluminum. I know frames differ greatly, but overall it is my understanding that steel is going to flex more. I prefer that in a road bike with narrow tires where I want a nice smooth, comfortable ride. I don't want that in my touring bike that's going to be bending around under the weight of the panniers/gear. I don't like my bike flopping around when I stand to climb, etc. As mentioned, the higher volume tires will smooth out the ride. And contrary to what others have said, I do think the weight matters. Why would people be talking about trying to reduce pannier weight because you're not hauling as much weight, but the bike weight not matter? Cutting 2-4lbs is cutting 2-4lbs no matter where you cut it. And if I cut 2-4lbs from my frame, then some wheel weight, then another 5lbs from my gear weight, I'm getting some noticeable gains when you add it up. I'd rather haul 10lbs less over many miles, up hills, etc. I also ride my touring bike as my main road enjoyment bike on the local bike paths, so it's nice to have a lighter, faster bike when it's not loaded with touring gear.