Hi,
for projects with unexperienced peoples on line voltages: I would ONLYan ISOLATED source select!!
These is practically a conventional transformer=bulky + expensive, but practically disturbance less!
Trafo in the switched voltage converter as isolating element; is practically an Flyback architecture, their are smaller, cheaper & similar on the efficiency, but generate some EMC/EMI... I would select these way!
If you select the switched converter solution, is in my opinion a plus trafo as "Secondary side" element irrelevant, but maybe other specialists are on other meaning?
Why; if its on the AC side(Output) of the switcher= you have line frequency transformer to apply, same as a normal isolations trafo=bulky + expensive & why did you have maked a plus switcher into the system? Their can be isolated too!
K.