BoomBoom:
dont forget the fireborn have frenzy as well. they will have 3 base attacks, blazing body, and stomp until they are beaten in combat.
khedyarl:
Fireborn don’t have frenzy. Only the destroyer has frenzy.
fattdex:
Bull Centaurs just ‘work better’ in my experience. Only two wounds annoys me with fireborn, a lot of burning bright casualties before they see any chop annoys me with fireborn. The only effective way to have them costs the same as a destroyer annoys me with fireborn.
Vogon:
I�?Tve had great success with both of these units (even using just 3 Fireborn) the local meta is for smaller games usually around the 1000pts mark with the occasional big game at around 2500.
So far I�?Tve only run Bull Centaurs as a unit of 3 with hand weapon and shield but use full command and warbanner. The high toughness, 2+ armour save and the 6+ parry makes then an extremely resilient unit capable of holding up units for quite a while. They may only be dishing out st4 hits but that�?Ts enough to deal with most of the infantry units that they should be targeting. I hope to buy another unit of 3 once the Taur-Ruck is released by FW giving this unit a rank bonus on top of everything else.
The Fireborn are good but need to be used wisely. In lower point games I�?Tve had three of them wipe out a unit of Swordmasters (before the new book) in a single turn of combat losing only one model in the process. As the St4 flaming attacks go before even the old Speed of Asuryan rule it�?Ts possible to decimate small elite T3 troops before they get a chance to do anything. The long charge reach is pretty handy for this 
Again they need to be targeting infantry to make the best use of them but with their higher than average strength means they can take on at least light cavalry and stand a good chance of causing decent damage.
They realistically need to be in units of 6 to avoid the worst of a failed toughness test. Three is just too risky (even though I�?Tve had some good results using 3) and they tend to die before the end of the game giving easy victory points away.
If I were forced to choose just one of the two units I think I�?Td be hard pressed unless I knew what I was facing but I think I�?Td probably go with the Bull Centaurs, mostly from an aesthetic point of view. I just love the models
Cheers Vogon
Baggronor:
These two units have different roles, they’re for doing different things. A straight comparison between them is like comparing a table to a bed and asking which is better.
Fireborn are for mowing down infantry fast and killing enemies with regen.
Small units of BCs are flankers, big units are a mobile roadblock, Tau’ruks turn them into a mobile hammer.
‘Which one is better’ depends on what you need.
Bolg:
These two units have different roles, they're for doing different things. A straight comparison between them is like comparing a table to a bed and asking which is better.I vote bed! I can work in bed, I dont really like the idea of sleeping on the table d:
Baggronor
But seriously its a fair question to ask with one, the are both special choices and cost about the same, good chance you want to bring either unit to a tournament or something.
But yes in the end its what you need or what fits your playing style.
Geist:
I vote bed/table, and yes those exist and they are fairly cool having had one once. That is the problem with the two units, they can not be called on to do the same task as one another. Other armies have access to multi use units, we dont really have that.