I'd stay away from the CS Nightshade push dagger. I saw a review of it once and the tip had broken off when the reviewer was removing it form the package.
The CS Urban Pal is a bit small, but that might actually be a pro. While it could certainly kill if you slashed an artery or something, a blade of that size is less likely to do serious damage. Legally, this would be to your advantage if you ever actually had to use it in a SD situation, but keep in mind that any use of a knife, even one that small, will still get you in serious trouble. You'd need to prove beyond a shadow of a doubt that you felt that your life was in danger and that you only used it as a last resort after all other options had been exhausted.
The Comtech Stinger may be the best choice. Being the least lethal of the three, it's the least likely to get you in trouble if you have to use it (though if your jurisdiction thing it's too much like a knuckleduster, you might still have problems). It would also be the easiest to deploy given its size and method of operation.
It's all a balancing act between effectiveness and consequences of carry/use. Given the confusion that is often involved in tense situations, you also need to consider the possibility of misinterpreting the situation and using force where it is not warranted. With that in mind, this my person preferred order, for best to worst:
1. Comtech Stinger - One good punch can buy you enough time to escape, and both carrying and using one of these should carry the least severe consequences of all three options.
2. CS Urban Pal - As small as it is, it's still a knife. While it would be easier to end a fight with the Urban Pal, the consequences of using it would be pretty bad. In all but the most serious of situations, you'd probably be better off using your bare fists than a knife. That being said, the diminutive size of the Urban Pal makes it less lethal than other knives while still giving it enough utility to end a fight.
3. CS Nightshade - I don't know if it was just that reviewer's product or what, but if his tip broke off just removing it from the package, I wouldn't trust it in a SD situation. For that reason, I cannot recommend this one at all.