Work requires energy. If you recall you basic high school physics, matter cannot be created or destroyed, only changed from one form to an other. Boil water and you will lose some of that water -- a licquid--into air in the form of steam. A transformer will "waste" some energy in the form of heat and some more energy in the process of generating a magnetic field ( in the case of a magnetic transformer ). I'd not call this waste as the cost of the work the transformer is doing. Every transformer has a VA rating and you can substitute the specific, accurate VA rating for the specific transformer rather than use the generalized, assumed value of 20%

I do not know much about the relative efficiencies of electronic vs magnetic transformers. Electronic transformers are smaller and so very popular with many fixture designers. They cause less lamp buzzing issues as the transformer itself does not make any noise. On the other hand electronic transformers are typically much less reliable, especially as 98% of them are made in China for the least amount of money. They are much more difficult to dim, typically due to the reverse phase dimming and capacitive load they present to the dimmer.

I simply do not know the aswer to your question though if you look at the VA rating on the transformer you can make your own determination. My gut guess would be that electronic transformers are more efficient and require less energy to the work of transforming 120v to 12v than a magnetic transformer.