Well in photoshop you 'can' tell the difference between png and jpeg. I know cause I stopped using jpeg for that very reason unless I am not using anything that benifits from png.

If you make use of shadows, glows, transparancy, etc PNG absolutely mows down the competition.

Sharp edges and text is also better handled by png.

Just about the only thing that out performs PNG is GIF when it comes to animation and JPEG when it comes to the final save of photographic data(which is what JPEG is made for last I heard).

Therefore if you make heavy use of shadows, glows, transparancy and those sort of special affects PNG can make a very obvious difference(I noticed it purely cause I use those effects alot, then I researched it and found out why.... PNG is best for anything that isn't a photograph in nature).

So that is the question. If you make heavy use of the affects above listed as being PNG's specialty you likely just want to aim for smaller images using png. If you don't use those effects heavily then likely the difference will be small to nothing so using a smaller format would be wise(as long as it is a high quality jpeg).