I once had the same problem having to chose a format. That's when I compared tga and tiff. I don't really know which one is "technicaly" better compress or anything, but when I was experiencing with them I found that tga gave me some trouble with the alpha channel, using it with photoshop, and the quality seemed better with tiff than with tga. I think the size was also smaller as tiff when I didn't had layers.