If I have only have six sprites for my platform images (TopLeft, TopMiddle, TopRight, BottomLeft, BottomMiddle, BottomRight), am I better off using 4 sprite instances (TopLeft, TopRight, BottomLeft, BottomRight) and putting the "middle" pieces into two TiledBackground objects, or leave them all as sprites?
It depends whether your project is optimized for performance. Just bare in mind performance is prior to the amount of objects used. Take a look at this page: scirra.com/manual/134/performance-tips
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You may also consider using the 9-patch object for your needs. Go check it out in the manual--it may help you quite a bit,