Use Both
Use both Amplify Damage and Decrepify where you need to. Decrepify is most useful for act Bosses, where you're going up against one particularly powerful monster.
As with previous posters, my advice is to make Amplify Damage be useful - 3 yards radius is no good for me because I'm on a high latency link (modem, in Australia), so I need the extra radius to counter my useless mousing skills.
Finally, don't always make it a "AD or Decrepify" choice. You have other curses such as Attract, so make sure you use them. Amp the crowd, Decrepify the unique, then attract on the guys in back. Failing all else, you'll attract monsters from offscreen and make the crowd big enough to warrant a CE or two :clap:
My necro seems to manage okay against everyone except Diablo and Baal, with natural skills only in Summoning. All other skills are given by items and boosted by +All Skills items. This build is workable in Normal, I haven't been to Nightmare yet, since I keep being PKed by punks in the Throne of Destruction. I do have to do the usual prebuffing of my skeleton army with a skill shrine and special gear, but that's just life as normal isn't it?
Eventually I'll have a write-up for the "lag resistant necro build" :thumbsup: But that will come after I defeat Baal in Hell.