I've been pondering how useful the various buffs are when applied to different units. As in, if i have various characters handing out reroll 1s, reroll everything or +1 to hit auras, who gets the most benefit from what.
A couple of things are obvious: reroll all is always better than reroll 1s (except when you hit on 2s, when it's a wash); +1 to hit is always better than rerolls 1s (because one face producing an extra hit is better than one face producing the chance of an extra hit).
But are there any situations where it's better to throw buffs on one set of troops over another?
Here are basic chances of hitting naturally, rerolling 1s and rerolling everything (all given in 36ths, for ease of comparison).
Reroll 1s is simple - the better your chance of hitting in the first place, the more you benefit in absolute terms. However, you always gain a 17% over the base roll. That is, 2+ benefits more but was hitting more anyway, whereas 6+ benefits least but even a single extra hit is a significant increase.
Reroll everything is more interesting - models that have a better chance of hitting still benefit more from an individual reroll, but are less likely to generate those rerolls in the first place. For example, 2+ to hit with full rerolls gains 5/36 hits, or just 17% more than they would without rerolls (makes sense - full rerolls is the same as reroll 1s to a 2+ shooter). But 4+ with full rerolls gains 9/36 hits, a full 50% more. And while 6+ only gains 5/36 (same as 2+), that represents an 83% increase. Overall, 4+ gains the most potential hits, which might make it a better target for full rerolls than anything else.
I've already pointed out that +1 to hit is always better than reroll 1s. But what about full rerolls? Turns out that full rerolls are better than a +1, unless you hit on 6+, in which case it's marginally better to hit on 5+ base (11/36 vs 12/36).
So broadly speaking, full rerolls beats +1 to hit, which beats reroll 1s. So far, so good. But this might help us determine where to put various auras. Let's imagine I have a unit hitting on 2s, and another on 4s - which should I buff with my full reroll aura?
Here, the +1 unit os on the left, the reroll unit along the top. In every case, it's better to improve the ballistic skill of the less accurate unit, and reroll the more accurate one. Improving BS is a steady progression, worth 6/36 each time. But rerolls 1s is worth as little as 1/36 to 6+ units, and only 5/36 to 2+ units. That being the case, you get best value by maximising the reroll bonus (sticking on the more accurate unit) and putting the fixed bonus for +1 on the other unit.
Now, I suspect that all I've done here is tell people what they already knew anyway. But I enjoyed playing round with the numbers, so there's that.