Blissey / Chansey and Togekiss play very different roles on a competitive team. Blissey or Chansey can pretty much sponge any special attack thrown at it, and retaliate with a status move or a Seismic Toss, which will almost guaranteed do more damage to the foe. If a Blissey Holds a Chople Berry, counter suddenly becomes a great option. It will normally KO the foe unexpectedly, and possibly ruining a strategy or synergy on the opposing team. Blissey also works as a cleric with Heal bell and a great Wish passer, two unique traits Ferrothorn would kill to have. One more great thing about Blissey or Chansey is that they have instant half health recovery in Softboiled, which makes their life span much longer.
Togekiss plays more of a special tank roll on a team. One of the very unique and extremely useful qualities it has it a very annoying Paraflinch combo, a property that it only shares with Jirachi, which is OU. Togekiss being the only UU paraflincher with the Serene Grace ability makes it an all star for a stall team. Air slash, Thunder wave, and Serene Grace is the dreaded combo every team fears. Togekiss's stats are excellent, boasting an especially high special attack and special defense, while still having that physical bulk that would make Blissey way too overpowered. Togekiss also works as a cleric with Heal Bell too. If a Togekiss is leaning on the offensive side, it has the movepool to support it well, with a moveset of Thunder Wave, Air Slash, Aura Sphere, and Heal Bell or another attacking move that provides types not yet covered.
Sorry if this is kinda long, I do this when there are topics that I know lots about :P Hope I helped you make the right decision :)