gif files will tend to be a little smaller since they don't support a transparency alpha channel (and maybe for some other reasons). Personally, I don't feel the size difference is really worth worrying about nearly as much as it used to. Most people are using the web with some sort of broadband now, so I doubt they will notice a difference.
It's probably more important to use the type of images that your manipulation tools work best with.
Plus, I like the ability to put an image on any background and have a drop shadow work, which points me more towards the png format.
