Either one works the same as far as quality and file size I believe. One for sure has an alpha channel, maybe both...that may be the difference. Check on that.
I personally use really large .jpg files. But that's my preference. If I was doing a large animation, with lots going on, I would dumb down my textures....
http://www.mayang.com/textures/
To get an idea of the textures I use go to this link. Of course most of these textures need to be re-leveled and offset for tiling...A lot of these textures are 1600*1200--which is gargantian....huge. But they look really nice, expecially tiled.
If you don't know how to do that...go here...
http://grayson.cgworks.com/tutorials/tile/index.html
Take care.
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