You can't get Togekiss in Diamond/ Pearl until after you've beaten the E4 and got the Pokeradar.
Therefore there isn't really a choice to be made if you can't trade from HGSS or Platinum.
To be honest, they both serve really different roles. I would go for Togekiss (with Serene Grace) if you need a fighting counter, Magnezone if you need an Ice/ Rock/ Electric resistance.