Join Date: Jun 2005
Bikes: 1990 Trek 1500; 2006 Gary Fisher Marlin; 2011 Cannondale Synapse Alloy 105; 2012 Catrike Trail
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Butting doesn't make a tube stronger, it makes it lighter for the same given strength. For example, if two frames, A & B, are equally strong, the one with butted tubes will be lighter. The problem with taking that information and applying it to real world examples is that you probably won't be able to determine whether the two frames are equally strong in the first place, without being a metallurgical engineer, or having one as your best friend. If you can find some manufacturer-provided info about the frames' load-bearing capacities (and you deem it credible) and they're comparable, go for the butted version.
It would be easy to make a decision about this sort of thing if we weren't Clydes, who are forced to play on the extreme edges of frame structure. In the fly-weight world, it's generally considered to be true that straight-gauge tubing (or heaven forbid, pipe) is not as high-quality as butted or double butted tubing. But as soon as you start dealing with 250-400 pound loads, those assumptions go out the window.