Totally agree on the welding over fasteners regarding strength. When a weld is properly executed there is no gap or seam. The two metal elements are fused as one.
That said, there are metal components on buses that should not be welded on due to tempering issues...such as the frame. GM and other have produced and distributed a number of safety bulletins outlining the proper procedures for working on frames and frame members which specifically state that no welding should be performed on their frames. There, you want to use appropriate fasteners. Hot bucked rivets being the first choice there. Also, any forged items (like older front axles. They can be welded, but unless you are a very skilled metal worker and can properly re-temper the metal, it is not recommended.