Wow, holy freaking tangent.... I never considered the Gauss rule even close to the Rend rule (because it is not)... Read your Codex, apply the rules as they are writtin, and never trust an opponent... Its OK to say things like "I don't know your codex rules, could you show me ?"
Now on to the old topic of Immortals vs Detroyers.
They are different, neither is better or worse as they should be used differently and have different priority targets. Think about it.... If your using your Destoryers to shoot the same stuff your Immortals are shooting (all the time) then you not fighting as effectively as possible. Here is an example for you. Start a unit of Immortals (7 = 196pts) and a unit of Destoryers (4 = 200pts) 36" apart form each other and see who wins that fire fight... now start them 12" away from each other and re-roll the dice... Do you see the differnce now... Destroyers are meant to work the flanks of the enemy from range... Immortals are meant to slowly entrap the enemy as they reduce the range and fire on them. Neither one is better, they should be doing different jobs within your army. Consider other factors as well, see how many shots from Immortals it takes to kill a single Dark Eldar Grotesque; its impossible as you must have a str=6 or better weapon weapon to kill Grotesques (read the DE Codex, although Rending would work if Necrons had it). So if your facing off vs DE and they are using Grotesques, then you better have a Destroyer squad on hand, or your going to loose and never cause any damage what so ever. I can give you examples for using both units, but my point is that you need to balance your army list, so pick the unit that you need most, and stop worrying about what everyone else is doing.
PS... I use both Immortals and Destroyers in most of lists.. I also use H-Destroyers, Wraiths, Flayed Ones, Monoliths... hell the whole army selection as my lists change and some units work well together or on their own.
CaHG