First, don't get confused between butted frames and butted tubes. All high quality steel frames will have butted tubing. Butted frame tubes are thicker at the joints for strength. Unbutted tubes are joint thickness end to end so they are much heavier.

Back to your question; in general lugged frames are stronger while tig welded frames are slightly lighter. Lugged construction allows frames to be brazed at lower temperatures which means that joining process has less effect on the metallurgical properties of the frame. Air hardened steels have somewhat mitigated this issue, though.