Shaggoth or Giant

The Shaggoth and the Giant are both very situational pieces that I would not use in any competitive setting. If I want to have a nice fun game, I'll bring the Shaggoth. If I want to have a hilarious time, I'll bring the Giant and try to mimic whatever roll I get from him. If I am playing a game that is part of a tournament or league, I will usually leave both at home and either bring the Hellcannon or another unit of knights.

Shaggoth:
Pros:
Great Weapon makes him Strength 8.
Yes, but S8 is only more effective than S6 if you are going after some serious toughness or very good armor save.
Scaly Skin and Light Armor gives him a 4+ Armour save
Without a ward save or regen, this is almost worthless. This is not a pro, especially for a large target.
Immune to Lightning and can gain Frenzy
Unless you are rolling with Kholek, I highly doubt you will ever get shot with Lightning.
Cons:
More expensive
"Very expensive" would be my phrase of choice.

Giant:
Pros:
Interesting attack options
Only interesting in a non-competitive setting.
Can be marked for mild customization
MoS is fantastic, by the way.
Stubborn and Leadership 10
Cons:
No save whatsoever
Low/no armor + large target = death to a shooty army.

Same:
Tough 5, 6 wounds
ITP, Large Target, Terror

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To clarify my first paragraph, I would never bring the giant to a competitive game. As for the Shaggoth, I would probably bring him if I could maximize his strengths. This means I would use him if I knew my opponent would not be immune to psychology, would not have ASF, would not have heavy shooting at S4 w/ AP (so no guns) and definitely not a lot of D3/D6 wounding shooting. As you can see, this cuts things down my options quite a bit. Of course, if one of these elements was present I could probably work around it pretty well, but of two or more would, I would be seriously hurting.

Sometimes I like having a Shaggy:

-Against another WoC army. Seriously this guy was built to take our own army out.
-If I knew my opponent would be running an independent character outside a block of regular infantry/calvary.
-It was a larger game where a nice hunter/killer could be used effectively and have enough screening from opposing charges.

Anyways, I hope this helps. Honestly, the Shaggoth is a great model and very fun to paint. I do own one and love it and would never get rid of it, I just don't see it being very practical in a tournament setting. However, in a fun game my opponent knows there is a very good chance of Shaggy making an appearance, and its always a riot when he does show up.