MTBs were welded because the lugs available at the time did not permit the desired angles. The technique stuck and became std. Manufacturers like reynolds then made tubesets with shorter, fatter butts more suitable for welding.
Both techniques work well and are well proven in the most demanding conditions, eg extended expedition touring. Rust is not a big issue with good steel frames, you can use wax or oil protector such as Framesaver. Drainage holes in the BB shell may help. I kept my old chromoly roadbike outside in a damp, coastal town 24/7 for 2 years.