Dreads and Kans
DREADS & KILLA KANS
Overall, they're very similar units, no matter what anyone tells you. Since this is somewhat a point of controversy, let's look at their stat-line:
The Deff Dread is a heavy choice walker with two guns, armor 12/12/10, 3 base attacks (+1 for extra CCW), initiative 2, WS 4, BS 2, and hits with strength 10, no saves allowed. The dread can also be taken as a troop choice, if you're using a big mek.
The Killa Kan is a heavy choice walker with a single gun, armor 11/11/10, 2 base attacks, initiative 2, WS 2, BS 3, and hits with strength 10, no saves allowed. The kans can be taken in groups of one to three.
An important thing to remember is that unlike the dreads of other armies, Ork dreads have no way of deep striking or getting into enemy territory quickly (besides flank march in Apocalypse games). Similarly, unlike similar walkers in other armies, Killa Kans do not have the 'scout' ability. With 5th edition, both the deff dread and the killa kan will be able to run, which will help, but they have no particular way of advancing quickly. Because of this, both Deff Dreads and Killa Kans are defensive units in nature. Even if you march them up towards your enemy, it will be a relatively slow and steady march at best.
Like most ork vehicles, you can choose what weapons to equip the walkers with. They both can choose between big shootas, rokkits, and kustom mega blastas. The deff dread can also buy scorchas and additional close combat weapons, but the CCW come at a very high cost, and at the expense of shooting. The Killa Kans have the grotzooka option, which is a relatively strong, yet short range blast weapon.
So which should you use, Deff Dreads or Killa Kans? To answer that, let's look at how they perform in each of their respective rolls:
1. Wall for the rest of your army
The Killa Kans actually do a better job simply because they cover more area, when you purchase 2-3 per unit. Granted, the Deff Dread has better armor, but against powerful anti-armor fire the difference is negligible. Lascannons and railguns aren't going to be significantly impaired by 12 armor.
Fielding multiple kans per unit can be a disadvantage against some armies, however, as powerful guns that fire more than once (assault cannons, auto cannons, psycannons, ion cannons...most 'cannons' actually) will be able to hit multiple kans per turn.
In any case, vehicle units always work best when they're fielded as few per unit as possible, especially in Apocalyspe games, where there is no unit limit. In those games, a wall of 6-9 kans can be really tough to deal with, given that each kan has to be fired at separately.
2. Shooting
Again, the killa kans come out on top, mostly due to their greater BS. Deff dreads do have twice as many shots each, but at a decreased BS, and twice the unit cost. Going by actual hits, killa kans are ahead. Some prefer dreads with scorchas, but I find getting flamer templates within reach of the enemy is nearly impossible without a quick method of moving around.
3. Assault
Dreads are ahead, but only marginally so. Both have the same strength, basic troops won't be able to hurt either (unless they have grenades), both are low initiative and powerklaws kill either just as easily. The only differences are the Dread's higher WS, and number of attacks...and even that advantage disappears when you compare them point to point, since you can buy 2 Kans for the cost of each Dread, and each Killa Kan would get a bonus attack from charging, giving 2 charging Kans 6 attacks, whereas a Dread only has 5. To make the difference even smaller, against marines, both the Kans and the Dread hit on 4+, despite their difference in WS.
4. Filling the army
This is where the dread truly shines. Unlike the kan, the Deff Dread can be taken as a troop choice, as long as you're using a big mek. This frees up points for elite, fast attack and heavy choices, which are very popular in many ork armies, and frees up heavy slots for more walkers, wagons and big gunz units. Killa Kans always take up space, which keeps them out of many lists.
So which should you use? If you can only take one type, but price and force organization slots are not a problem, then I'd say 'Killa Kans'. They're better at shooting, more difficult to completely wipe out, and provide more bang for the buck than a Dread. However, if you're looking for an inexpensive troop choice, deff dreads work well, as long as you were planning to use a big mek anyway.
The ability to take Deff Dreads as troop choices also enables you to field 'walker' armies, with 2 or more dreads, 9 kans, 1-2 Kustom Force fields, and if you like, an SAG and/or some Lootas. Just be sure to give the list at least one KFF, or else 'shooty' armies will tear your front line to pieces.
The only thing I need to add is that with the exception of 'grot riggers' for Dreads, the walkers should never be given any extras. The reason ork walkers are effective is because they're relatively cheap for what they do, especially the Killa Kans. If you really want to load up your Dread with high cost weapons, armor plates and several extras, than you're playing the wrong army. Marine and Chaos dreads are powerful enough to warrant the upgrades. Ork Dreads, on the other hand, barely warrant their base cost.
Here's the way I'd play them:
Deff Dread with grot riggers, and either 2 big shootas or 2 scorchas (I prefer the shootas)
1-3 Killa Kans with either big shootas, rokkits, kustom mega blastas, or grotzookas
Above all, remember: Ork walkers, like all ork vehicles, are far from invincible. Try to keep them out of harms way, and don't overspend on them, or else they'll never make back their points.