1) The tried and true CO Bus. Usually you're looking at 1-2 Oldbloods alongside 3 Scarvets leading the charge with around 10-13 CO in support. I've seen a lot of lists be very successful with this build as it relies on a fairly impervious wall of 1+ save characters. However, this unit usually costs 900+ points and it's stupid. I also find that, though tough as nails, 2 wound models don't live forever... 2) Something I've got the models for but haven't tried is the 8 ripper unit. This unit is 400 points and relies on having at least one rank of rippers left to fight by the time they get to strike. They are trying to assassinate enemy generals/level 4 casters. Flanks are nice but the number allows them to kamikaze into a front arc also. Not sure how well they'd do but they'd be fun. 3) I'm 2 Kroxigor box sets away from a full 18 krox horde. I've done the math and (minus chariot impact hits) this unit can survive even charging into the front of a 30-witch elf horde with the khaine-chariot-thingy. The math works out that the witches generally kill off the back rank and then lose almost the whole unit. The kroxigor striking at regular witches win on combat res the first round and, if they stick, the witch elves only have about 10 left. Thoughts?
I'm going to assume that your talking about including such a unit in a list of between 2200-2600pts. In this case, I would argue that the CoR unit is the best, with those supporting characters you've talked about. The reason why? - Options. 1) If facing an opponent that can literally cream the unit you are bringing in support, solo-charge one of the characters into said unit. Ideally, one of the characters even has a 2++ flaming ward, should you be so lucky as to have to deal with a unit with flaming attacks. 2) Tough opponent? Make way with a wall of characters. Not so tough opponent? Do it anyway, then reform after round 1 to bring more Saurus Cav to bare. 3) Swiftstride. Either your hammer should have Swiftstride, or you'll want to bring in a supporting unit that does. Nothing worse than breaking your opponent, only for them to get away, then rally in their turn. Swifstride gives you options there - and also makes short to medium charges more reliable. 4) Back-up Hammer. Take a medium unit of Saurus. Look at how amazing it becomes if all the characters suddenly deploy in that unit and not your Cav (obviously, don't do this vs. Cannons and PSun). Or - if your opponent has no units that need to face such a heavy hammer, split the characters between the two. 5) Divide and Conquer - Though this advice is not the technical application of that term - you can split your characters off and have multiple combat threats. Against Wood Elves where the Waywatchers are engaged or dead - split those heroes out and have each of them take on an archer unit - they can do it! 6) Layered attack - there is value in having attacks occur at different Initiative steps. They don't always have to be at an Initiative higher than your opponent - but having 1-2 guys who strike before the others, then a majority who strike at the next Initiative step, and a few attacks at the ASL step, gives you the ability to direct your attacks when working to kill enemy monsters/characters/champions/etc. 7) Ability to cut through opposing defenses in the form of Flaming Attacks (via bsb), the OTS, and maybe the Blade of Realities (if you feel so bold). The other two units you've mentioned have none of that (except the Rippers have #3 and a bit of #6, and those two units also both Stomp - but you can't direct those). So while the CO bus may cost more points - there's a pretty good reason for that. Just my $0.02
+1 Ghostwarrior. However a horde of 18 kroxigor sounds sweet. Maybe not the best option but it definitely sounds fun!