So, what are your plans and do you know which Army you're fighting ?
The skull catapult is great for spreading panic amongst lower grade low leadership rank and file. It costs 90 pts but you should pay the extra 20 pts to lob the heads of your foe's race into his somewhat perturbed units. Doing this will force them to take a panic test with -1 to their leadership if they take any casualties.
The screaming skull catapult is ideal for targetting enemy artillery which doesn't usually benefit from high leadership nearby. Alternatively, it's great for hitting the enemies massed rank missile 'shield' units, especially troops like gobbos who are likely to run, maybe straight through the ranks of following troops causing 'domino' panic and maybe saving you from a fanatic attack..
The Bone Giant is a hardy foe, if a little unpredictable. Providing the Battle Standard is within 12" than he deducts 2 from wounds from any that were inflicted on him in a lost combat. This makes the bone giant quite tough to break down (he has 6 wounds).
To make the most of the giant you really must keep him within 12" of your General and Battle Standard. I guess this is a bit predictable from your enemies perspective but he's quite a large investment for a 2000 pt army so it's worth it.
The choice between these 2 is too hard to make without info on the rest of you forces and who you are facing. If you take the Giant you should also take carrion to engage any missile troops in his line of sight. He will fall to missile fire eventually if you allow your opponent to take this initiative.
