I mostly use a spear on characters on cold ones. Great weapons are the same as spears except they don't give +1 AS for a shield and they still give +1 S in the second and subsequent rounds of combat. Both are very good, and your models weapon could count as either IMO. BTW, that model looks awesome!
Mostly I take spears evt. Magic weapon myself. 3 pts for thouse shields are just to cheap to give away. Besides I often take enchanted shield if I go for a mounted character, making an awsome armor save on him
The problem with the great weapon on a mounted unit is that you only get +1 to your strength instead of the +2, always strike last (charge negating of course) and can't use a shield. A halberd gives you the same effects without the always strikes last, and a spear gives you the shield and the +1 strength on the charge. 7th Ed. really killed off the great weapon on a mounted unit (ask any Brett. player that had questing knights) but if you're really wanting to go with it, then don't let me stop you. It looks cool, but for game purposes I'd say to call it a halberd instead.
No, but you still at least attack on initiative order. If you want a shield, go with a spear. Hand weapon is the only other alternative and if you do that, you might as well make it magical, because there's no other benefit to a hand weapon while mounted.
But, you can take shield to be able to switch from offencive to defencive. Right? just as the temple guards...
from my understanding of rules... you can only change weapon setup (if they have more then one) when not engaged in CC
Actually, you can choose to use shields when you are under fire and halberds when in CC. But in close combat, at the beginning of each combat (not at beginning of each combat round) you can choose from one of the CC weapons, if they have many.