Is re-rolling fails better of +1 to the roll?

[Skasian]

My maths is failing me, I cannot work out if it's better to take a trait/spell that allows re-rolling fails or give a +1 to that stat?

e.g. Evocators have empower to re-roll wounds but they can also take Celestial Blades now which gives +1 wound instead. Which would be the better option mathematically?

[Yoshiya]

The only time that +1 is strictly better is going from a 6+ to a 5+ (the difference is 1/36 so very minor).

As an example, something hitting on a 4+ hits 50% of the time.

+1: turns to a 3+ meaning 66.66% hit

Rerolls: effectively add 50% of 50% (the original misses) to the original 50% meaning 75% hit

 

Note this is only true for full rerolls, the maths is more complicated for partial rerolls.

[XReN]

In that case - rerolls all the way

[Keldaur]

13 hours ago, Skasian said:

My maths is failing me, I cannot work out if it's better to take a trait/spell that allows re-rolling fails or give a +1 to that stat?

e.g. Evocators have empower to re-roll wounds but they can also take Celestial Blades now which gives +1 wound instead. Which would be the better option mathematically?

Except if you hit on 6s, rerolls are always better

 

Comparatively 

3+ to hit (+1 to hit gets to 83%) (RR to hit gets to 89%)

4+ to hit, (+1 to hit gets to 66%) (RR to hit gets to 75%)

5+ to hit (+1 to hit gets to 50%) (RR to hit gets to 56%)

6+ to hit (+1 to hit gets to 33%) (RR  to hit gets to 30%)