You need to fix your edges to get rid of the white stuff. The text could be changed too but I think that's your best bg so far from what I've seen from you.

As for the differences in PNG and Gif.
Gif- Smaller file sizes and lower picture quality then jpg and png. Gif transparency is viewable in all browsers I believe.

PNG- high quality pictures, larger file sizes and their transparency is only seen in FireFox and not in IE I believe. (Dunno about other browsers though). And PNG transparencies are easier to work with then gif.

It's a matter of preference really.