Since you guys only have the demo I thought I would give you a list of the special features of each unit from the manual. It may help you decide what units to pick in the skirmish.
1. Giant Rat - Lost Packmaster, Large
2. Clanrats - No special attributes
3. Stormvermin - Giantslayer and their halberds have piercing damage which helps against charges from knights and hitting the knights.
4. Gutter Runners - Skirmisher, Scout, Poisonous. (I think scout maybe broken because I can never seem to get them to be invisible to the enemy. The enemy always attacks them as soon as they are close enough. I may not understand how that rule works though.)
5. Plague Censer Bearers - Skirmisher, Frenzy, Disease.
6. Poisoned Wind Globadiers - Skirmisher, Magic Damage, Explosive.
7. Rat Ogres - Fear, Lost Packmaster, Huge, Cleaving Death, Crushing Blow.
8. Warplock Jezzails- Skirmisher, Armour Piercing.
9. Warpfire Throwers - Explosive
10. Warp-Lightning Cannon - Long Range Shot, Inaccurate, Demollish Walls, Explosive.
11. Screaming Bell - Fear
12. Warplock Engineer - Explosive
13. Assasin - Hidden, Poisonous
14 Warlord/Chieftain - None
15 Grey Seer - None
Definition for the attributes
Skirmisher - A skirmishing regiment is a small group of warriors fighting in a loose or dispersed formation rather than a formal ranks and files, enabling them to travers rough terrain swiftly.
This regiment Cannot be attacked from behind and con move at full speed on hard terrain. These troops cannot use formations and do not have charge impact damage.
Scout These troops are skilled at sneaking ahead to scout out the enemy before the main force arrives. Making best use of cover, or advancing at night, they are able to deploy ahead of the rest of the army.
The regiment is invisible until it attacks or the enemy regiment approaches within the immediate range. They can easily avoid combat and are subject to the anti hit unit rule.
Hidden Whether by mystical means or simply camouflage, the warriors in this regiment can blend into their surroundings, making them virtually undetectable until they choose to attack.
The regiment is invisible out of combat. They can easily avoid combat and are subject to the anti hide unit rule.
Fear Even in the war-torn Warhammer world, some foes are simply so noisome, unnatural or horrific that they instill fear in the enemy, making them a daunting prospect to fight against. Attackers lose morale when attacking a regiment with the Fear attribute.
Frenzy Certain warriors can work themselves into a fighting frenzy, a whirlwind of destruction in which all concern for their personal safety is ignored in favour of mindless violence.
The regiment cannot be fazed in close combat and is unaffected by any kind of morale effect. (great against fear causing units)
Lost Packmaster The beasts of this regiment are instinctive killers and should all the packmasters controlling them be killed they are likely to run amok, attacking both friend and foe. If monsters loose thier masters they become uncontrollable for the rest of the battle.
Armour Piercing Certain weapons, most notably black powder weapons such as handguns, are excellent at penetrating even heavy armour.
Regiments with this attribute reduce the armour of the enemy by 50%
Long Range Shot With these war machines, the crew do not have to see their target to fire upon it - they simply direct their war engine towards the place they think the foe is and let loose.
Such units can shoot further than they can see.
LargeLarge units are those that are bigger than man-sized, without being Huge, and such units cannot enter places where smaller warriors would pass with ease.
Bigger than humans, but smaller than giants, they cannot climb ladders, hide behind barricades or garrison into a tower.
Huge Some creatures are simply so large and unwieldy they are 'Huge' and this has a great effect on where they can roam on the battlefield.
Huge units cannot climb ladders, hide behind barricades, garrison into towers and are immune to charge impact.(that is the only difference between large and huge.)
Inaccurate Some weapons are simply too large and cumbersome to benefit from expert marksmanship, Their shots will usually deviate at lease some way from the intended target. Inaccurate ranged units' shots will be scattered.
Demolish Walls It is a strong weapon indeed that can smash apart the walls of a castle, while some monsters are also possed of such brutal power.
Units with this attribute are capable of doing Siege damage. Therfore, they are able to destroy castle walls.
Giantslayerssome units are trained or equipped to deal with larger opponents.
The regiment is more effective against large or huge units
Magic Damage The winds of magic course across the whole world, and they are sometimes bound to creatures or weapons to imbue their attacks with magical properties.
A regiment with this attribue does magic damage.
poisonous Some warriors of the Warhammer world use lethal toxins to envenom their weaponry, while others might use magical or holy substances that are effective even against daemons and Undead.
All attacks of this regiment have a 100% chance to poison the enemy. Poisoned models can die even after survivng a battle.
Cleaving Death These powerful creatures can smash into a horde of enemies, striking left and right with their deadly claws and fangs.
Regiments such as these are able to attack multiple units around them.
Breath AttackMighty dragons, along with a few even strager creatures, have the ability to breathe fire, noxious gas, lightning or even more bizarre substances.
Units with this attribute damage all enemies in a straight line. (The warpfire thrower should have this ability but in the manual it doesn't say it does. I think that is a typo because that is exactly the kind of attack they have)
Vulnerable to Magic The magical nature of some creatures makes them particularly vulnerable to attacks that are also magical in origin.
A regiment with this attribute is vulnerable to all magic attacks, including spells and magical weapons.
Crushing Blow The attacks of this type of regiment are particularly heavy and cause considerably more damage than those of lesser fighters.
Regiments with this attribute cause 2 damage with every successful hit.
Explosive Warpstone is an unstable elemnet at the best of times and using it in weaponry is almost an invitation for disaster - as the smoking remains of many Skaven have attested to over the centuries!
at 0 HP the regiment blows up damaging everthing around them. Regiments with the Explosive attribute are immune to warp explosions
Plauge As carriers of various diablolic poxes and diseases, the regiment may well infect those that fight against them.
This regiment can cause a plague to an enemy regiment hitting them in melee combat. Plague is similar to poison, but instead of afflicting only a single model, it damages the whole regiment for a short period of time.
These arn't all of the attributes but if you have any questions about a particular attribute or units just let me know and I'll write down what it wrote in the manual.
If you want to try out the empire side or chaos let me know and I'll tell you what attributes are for each unit.
Well I'm off, hope this helps.
. If they throw in Skaven as a race to play then I might just have to break down and buy it.
Some strategic running kept his blows on them and away from me. But, in the end, I had to put the poor bastard down myself. 
