The weight difference is trivial most likely something in the order of about 30 inches of steel 3.5 inches by .01" thick. Every little bit helps, but then if you are lineman like, you shouldn't have too much difficulty carrying that around.
The butting is mostly in the top and down tubes. The seat tube normally is externally butted and that may have been done since a machined tube helps with fitting the seat post and the outer clamp. The downtube is under tension and the top tube is under compression. For the same external dimensions the butted tube would be weaker, but it is butted because experience shows that the extra material is not normally missed. Under heat, the thicker ends are an advantage during welding or brazing temps. When bridging as if you sit on the top tube thicker ends are required just as a fishing rod is thicker where you hold it. An under compresion the butted tube has a slightly higher margin than an unbutted tube for remaining in collumn (but the walls of the straight gage tube are stiffer.
Overall, if you like the way the bike rides don't worry about it.