It entirely depends what you are looking to get out of your Saurus. Bare in mind that they will hit last and won't inflict many wounds (only 4s to hit - 5s if facing Nurgle Warriors, only S4), and Warriors have easy access to high Strength which will make a mockery of T4 4+. They just aren't very killy (for that you want Temple Guard) and aren't super-tanky, either. On the other hand, they'll be really good against any Marauders, which the Skink-Kroxigor will struggle with. You can only get 18 Saurus for the same cost as that Skink Cohort, but they are much better recipients of buffs like Wildform, so it does depend on what you want to build your list around. I think they're better at higher points levels when, like Temple Guard, you can start getting 24, 30 or 40 of them, which starts to offset their weaknesses (doesn't matter if you're removing handfuls of models - there's plenty left!) Entirely honestly, I'd suggest trying out both options and seeing which you prefer. Our Core options are so limited that it does tend to come down to a matter of personal preference. When the book first dropped I was on the Saurus train; now I'm back on the Skink Cohort bus so just be aware that this is simply my opinion which does often vary!
Cold One Riders have the same problem as Saurus: they have very poor Initiative (so stuff tends to hit them first), and they have very poor Weapon Skill (so not only do they get hit easily, but they don't hit things very often either). Sure, they have a metric ton of attacks per model which you would think sort of balances out. And back at the start of 8th, when everyone was chucking around hordes of low quality troops, they might have had a niche. Now the game is in a place where you have lots of T5+ and AS1+ on small, mobile units. This means that unless you are willing to fork out the outrageous cost to get Spears, they probably won't be able to achieve much. Especially against Warriors of Chaos.
So this goes back to the first point: not only do they get hit first, and don't hit very well, they also don't wound very well. And on top of that, because of the same proliferation of high armour saves, there's an equal proliferation of high Strength - which means that T4 2+ Armour Save whilst looking great on paper, actually crumbles as soon as it runs into the increasing number of S5+ attacks (of which Warriors have easy access to). This is the same reason that some people are switching from Saurus back to Skink-Kroxigor - if you're going to be removind models to S5/S6 attacks, you might as well have more of them to retain ranks/Steadfast (the Skinks) and a few that can do something (the Kroxigor S7 attacks), as opposed to stuff which is just dying and not contributing anything with poor S4 attacks (the Saurus).
With the COR, you're paying 30+ points per model for a unit which gets hit first, gets wounded easily, can't stand up to high Strength, and any surviving models won't hit back very well or inflict many wounds. Sure, like everything in the Lizardman book they can work if you really put the investment in (and buff them), but that's just the problem - everything in the book works like that, and because the book is internally balanced so terribly there's a bunch of stuff in that same 150-200 points bracket that will deliver much greater performance.
For instance, for the same cost as a unit of 5 naked Cold One Riders, you can get a Scar-Veteran on a Cold One, with a Dawn Stone, Dragonhelm, Light Armour and a Great Weapon. Now you've got 4 S7 attacks at WS5 (the likelihood is 5-6 with Predatory Fighter), and a 1+ re-rollable armour save, with the exact same mobility. No additional investment required. This guy is simply amazing - he can actually hit things, he can easily get through all that Toughness and 1+ Armour Saves that's about these days, and he's so tanky he can run in and take on entire units by himself (Yesterday I had one of these guys and an Oldblood cut through 18 Nurgle Halberd Warriors single handedly. Like cake) - up to and including standing up to Daemon Princes. You can give him the Other Trickster's Shard or the Potion of Foolhardiness for less than the cost of giving 5 CORs Spears.
On the other hand, 200 points is enough to get in a cheap Oldblood; 126 gets you a Skink Chief on a Ripperdactyl with the Egg of Quango, Dragonhelm, Light Armour and Shield (for a 2+ armour save - add either Biting Blade or Luckstone); for 160 you can get 4 multiple attacks Armour Piercing re-rolling to hit Flying Ripperdactyls (who will do much better than COR with WS/I buffs), 6 COR with full command and Spears is only slightly less than an Ancient Stegadon with Sharpened Horns, and the full 10 man COR unit competes with 8 Kroxigor. In all cases, I think that the alternatives are superior.
There's a place for CORs as Scar-Vet delivery systems (so he has a Look Out Sir), or in a minor role as flankers to support your combat blocks (and prior to Rippers they were a good way to get some Swiftstride to run down fleeing units), so by all means give it a try. They could work well in the aforementioned Wildform lists (go double Skink level 1, give one the Forbidden Rod to get some Power Dice at the moment you need it). I keep writing them into my lists every now and again just to see if the theory matches the practice, but I just think that - like Saurus Warriors - you need to manage your expectations.
