my experience of rippers has been in conjunction with the skink patrol (or shadowstrike starhost) formation - even in a unit of 3 these chaps are amazing. The ability to swoop down in turn 1 behind enemy lines and hit them before they get a chance to do anything about it is immense. I regularly cause 20 wounds on a unit when i do this. Steam tank, War Altar, High Elf Prince on a dragon, Warboss on a wyvern, Settra, Leon Leonceur, Durthu, 15x Black Orcs - All of these have died in a single turn to my Ripper-assault
Especially when your attacks are 9x claws 3+ to hit - re-roll, 2+ to wound - re-roll and 9x beaks 4+ to hit - re-roll - make extra attacks, 2+ to wound - re-roll. It's horrible, but a true alpha strike unit.
They will die immediately (mainly because my opponent just concentrates everything on them and they are always out on their own with no support) but that doesn't matter - they make their points back and I get to roll handfuls of dice.
Terradons - honestly, I stick them behind a piece of terrain and leave them there. The exception to this is when there are small, lightly armoured skirmishers, or lone wizards for them to eat. Then on the last battle round (or as late as possible) they fly onto an objective, drop rock on a nearby enemy unit and claim it. They don't have the longevity to last long out in the open or get dragged into a long combat.
If you are thinking defensive, go terradon. If you are going offensive take rippers and try to complement them with a unit of 10x chameleons (or two units of 5 for the formation), popping up in terrain for a 3+ save.