The advantage of the Halberd is two fold for Empire Troops as a staple choice. In any review of our three staple core block choices, halberds, sword or spear you will find each has their own supporters and detractors.
However the benefit of the Halberd is as follows as most people have eluded too here. Its the added Str of the swing without the I penalty that Great Weapons suffer.
The detachment system itself lends to an anvil style block in general. You have two choices, advance with them and catch a unit or stand still and invite the unit to your halberd, spear or sword points. The detachment system works by keeping two units close enough to either present another target OR to become the flanker itself for an attack. If the opponent attacks your block the flanker hits the side, if he goes for the flanker unit you could either block and then smack him with the main unit or flee with them so that they are now lined up perfectly for the charge.
I have found Free Company to be the best close combat style detachment to use in about a group of 15, so you can suffer a few losses and still be a flanker worthy of doing damage. As for the main block of troops to use, I tend to think the Halberd is better. Its a fact of the game that Empire State Troopers no matter what they are are fairly expendable, a ward save or parry save for me is not worth it on such cannon fodder, I would rather be striking back with superior strength. When you look at the more elite armies out there that Empire face (DE, WOC, Demons, Beastmen in some forms,) The extra point of strength is usually more useful than the save. I have resigned myself to the fact I will suffer casualties as Empire in the State Troop corps, so you compensate with numbers.
If the Halberdiers have a detachment of Free Company and a detachment of Handgunners you are covering a lot of ground fairly cheaply and effectively, with a decent character like a priest or a good support wizard with spells, your halberds become even better.
In general though.... the three choices you have all are ok, none are terrible for their use as outlined above. I just personally feel that strength 4 gives you so much more ability for a cheap unit than either a save or the extra from spears.
From a fluff perspective, every standing Counts army is under orders to maintain a unit of Halberdiers. Whether or not this influences you is up to you as well. The only thing I will say though..... THEY ARE THE WORST TO LINE UP IN REGIMENTS!!!!!! I spent hours putting them together then had to change them and move things etc..... the sword and shield option is always the easiest.... hmmm I need 10 wide by 5 deep and I am gluing them all tonight option!