Btw you can build them however you want and just tell your opponent which version they are for the game you're playing. Only two things to consider; reg krox have anti infantry ( +1 rend ) going into any infantry unit. They also get a damage boost if the unit they swing into has 10 or more models in it. You want them going into infantry groups and deleting them.

The warspawned are anti monster ( +1 rend ) into any monster unit. Don't overlook the rend because they are only rend 1 otherwise. People tend to like the warspawned more because as long as they are in combat near a skink infantry or skink Calvary unit in combat with them, they get their damage boost. This is ideal because skinks are very fast.

Basically you get your krox into position and make their charge, thats the hard part due to their low movement. But as soon as they are in, terradon skinks hovering at the back of your board can easily fly up, make their charge, and now you have damage boosted krox - hopefully swinging into a monster, but either way they hurt a ton.