Dînadan:
By the way i just found the part of the errata about the save it's at the page 2 of the errata of the rulebook on the left side. They confirm that your save can go below 1+ before any modificators are applied (0+ or -1 is possible for example). Which makes a hudge difference in the situations where you fight vs high strengh characters/units
vs Strengh 6 a 1+ armor save would moove to 4+
vs Strengh 6 a -1 armor save will moove to 2+
really not the same situation at the end! So 0+ matters in the case of the taur'ruk!
Spencer
Umm, I think you've either completely misread the Errata or are looking at an outdated one. The current errata (on the BL site) says pretty much what I said above - you can have items that can make it better, but it's capped at 1+, meaning that even if your items could give you a 0+ or -1+ save they still work like a 1+ and negative modifiers will effect them exactly as if they were a 1+ save.
so let's move back to the topic! who prefers the 4 centaurs and who prefers the taur'ruk and why? (with the options that i proposed at the beginning of the topic)
Going with your layout I think the Taur'ruk may be the better option as he's more resilient (as they share the same Toughness and they're only 1 point of WS apart, the enemy will probably cause roughly the same number of wounds before saves on each, and with the layouts you've given, assuming no modifiers to armour saves, theoretically the enemy would need to cause ~36 wounds to kill the Taur'ruk but only 24 to kill the Centaurs. I won't bog this down with working out the odds with saves, but I'm fairly sure the Taur'ruk still comes out ahead).
Further, the Taur'ruk is smaller than the unit so is less likely to be hit with templates, not to mention it'd only take one hit from the template, whereas the unit can take up to four; and against cannon and bolt throwers the Taur'ruk only takes one hit whereas the unit can take up to two (only upside for the unit is that the Multiple Wounds rule won't wipe out the whole unit against cannon and bolt thrower).
The Taur'ruk also has a smaller frontage (and depending on how you field the unit possibly a smaller flank) meaning that less enemies will be able to attack, meaning he's less likely to take a wound. On top of that, he can challenge, potentially reducing the number of attacks against him even further.
The Taur'ruk is also more mobile, needing less room to manoeuvre and consequently less likely to get blocked by other units/terrain and thus less likely to fail a charge. He can also be positioned such that an enemy is blocked from charging him, but he is able to charge them through the gap in the blocking units/terrain. He's also less likely to accidentally enter dangerous terrain when wheeling/manoeuvring, so less likely to take damage that way.
Being a character he can also bunker down in another unit of Bull Centaurs for protection if needed which the unit couldn't do. Doing so he can pass on his higher leadership to keep that unit around.
Speaking of higher leadership, in the event he's out of range of the General's Leadership he's more likely to stick around than the unit.
There's probably more, plus cons to the Taur'ruk, but this is all what I could think of off the top of my head.