PNG or TGA32 for semitransparent material

Is it better to use PNG or TGA32 for semitransparent materials in Unity?

Since Unity converts textures into an internal format the choice is rather in what tools you use to produce the respective textures. I would opt for PNG as I feel that the support for it is wider, though it really is only a choice in personal preference as both formats can convey 32 bit RBGA color information.

I tried to make a material usin a Text in PNG, and it doesn’t work. Only a big dark square is shown.
Any ideas?