They both do the same damage, but of different elements. Icicle will do water damage, dark touch 3 will do dark damage.
Because of that, they're good against different type of monsters; if you're fighting water element enemies, icicle won't do as much damage (water on water) as dark touch would (dark on water).
Check your element table (Help -> page 9 or so) to see what's good against what. ;P
