rle8 is an 8 bit sprite, meaning that the spin can have up to 256 colors (we're talking binary here. 2^8 colors = 256)
rleD is a 16 bit sprite, meaning that the spin can have up to 65536 colors. In other words, your sprite will have better gradients, and can allow more detail. It's generally more important on the larger sprites where more detail can be seen.
It's good to do both because someone running a slower machine will have a much harder time running rleDs than someone running 1GHz dual G4s.
For a bit of clarification, spďns have absolutely nothing to do with rle8 and rleD. For that matter, neither do shäns. rle8 and rleD are for sprites.
Spďns and shäns are similar, except spďns are for weapons, planets, asteroids, etc, while shäns are for ships only. Since you're asking about planet sprites, spďns are the way to go. They can use pict sprites, as well as rle8 and rleD, so you don't have to convert them if you don't want to. Just keep in mind that Nova likes rle8 and rleD a whole lot more than pict, and probably uses more memory when you use pict sprites.
Matrix
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"Nothing is fool-proof to a sufficiently talented fool."
