Sounds like a weasel answer I know but it depends. I'd say whichever approach is chosen should be used consistently through out the system. There is a heavy movement right now for "minimalistic" UI where the embellishments and flourishes to make the UI look more "realistic" are eschewed and just enough is placed to provide hints as to what a specific element will do.
A primary example of this is Microsoft's "Metro" design but the example you gave from Gmail fits well here too.
It all boils down to a matter of personal taste. As long as you don't have flashing text and animated "in construction" gifs, you're probably safe whichever approach you take.
I can give a good example of what not to do right here. The majority of the UX site uses flat, minimalistic buttons except the "submit" buttons. They feel out of place in the context of the rest of the site.