Which does more damage as an 8 talent (all other things being equall) ?
Maybe I'm missing the point but those two talents aren't remotely similar.
The answer to "which does more damage" would depend on so many outside factors - game mode, hero, hero level, skill level, opponent, base design, etc -- that a comparison without a very specific case is useless. My opinion.
IM gussing your talking about AD, SD 8/8 will give more Dmg than ds 8/8.
If you just want to know which does more Dmg taking in the activation percentage, DS does more.
But I'm 99% sure your asking for archdemon so put SD on. Also, SD is the only that should be 8/8 for AD so you get 40% activation as ds already does 500k at 5/8 caped