So what are the big differences between KOW and WAB. Well first off the lists for ancients are not official lists. They have been generated mainly by 1 individual with some feed back. What else is different you ask. Well much like HC you don't remove individual models you remove units. The key state for determining this is Nerve. Which effects how resistant they are to damage and their training and discipline. Units are also classified in 3 ways, routed, steady and wavering. Routed is running, steady is normal and wavering is in the middle. Banners and musicians effect nerve along with the unit size.
Optional rules in the book include building, siege warfare and multiplier games. I personally like the siege warfare rules but have never had a chance to try them. Now KOW historical really reminds me to HC though the rules aren't really designed for it. But seem like they would work well.
As a more direct comparison on one of the big differences (POINTS) I will give details below. These lists come direct from my Romans tab. For WAB you are looking at 2505 points with the models I have and no upgrades. The main special rules for many of the models are Drilled, Stubborn, Veteran for the veterans and the legionaries can do a testudo. Testudo only confers a 2+ armor save towards missile attacks. Note that the legionaries don't have heavy armor. One of the biggest benefits for the legionaries is that stubborn means that they don't need a panic test for fleeing units, they can give ground after failing a break test and if they would automatically break they can take a test first to see if it happens. Drilled is great because this allows them to always make some sort of move for free. The General also allows units within 12" to use his leadership for leadership tests.
Now with the KOW list clocks in at 1305 points. So if you like large games this maybe the system for you. You will be able to field a ton of models. This is part of the reason I am looking at this system especially for smaller scales. Now special rules wise the closest list I think is the Marian Roman Army the list would be as bellow. Some of the models aren't used because of inability to field enough models.
Regiment of Legionaries with musician and standard x5 = 750points
Regiment of Veteran Legionaries with musician and standard= 180 points
Regiment of Auxiliary Medium Infantry with musician and standard= 105points
Troop of Auxiliary Medium Infantry with musician and standard= 85 points
Troop of auxiliary archers with musician and standard= 75points
Now besides the obviously huge difference in points there are only two special rules really and they effect the legionaries. Pike phalanx and Testudo. Testudo for KOW makes it so if a unit is using it if they get charged on the rear or flanks it counts as charging the front. Pike phalanx confers +5 attacks for regiments and +10 for hordes after any multipliers for charging and such. It also makes cavalry in this case have a -1 to hit. A further difference is their are no upgrades besides musicians and standards to units. The exception being the general who can take a horse. The general also has the Inspiring special rule which effects any unit within 6'. This rule makes the nerve test be re-rolled.
Now having not had a chance to try this rule set because of local resistance towards non HC or WAB rule sets. I can only give general opinions. I think this rule set would be faster overall due to less special rules and options. I also think that the KOW historical lists would allow for much larger/faster games. This I like because I prefer using all my models its part of the reason I have so many. When playing WAB the games are often slow due to individual model removal and all the special rules and options.
Anyway check it out for yourself and see what you think. Link for historical lists for KOW is here.