Calling all mathematicians out there. I'd like to compare these two abilities and which one is better. Early game and late game probably there is probably a clear winner. At level 8 and 16 let's compare. How much is an extra re-roll (Bit of Luck) worth (+3? +4? Or?). Sacred Touch we know is +4 bonus at level 8 and +8 at 16.
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A reroll is worth anything from +1 to +5 (equivalent) depending on the exact DC or other target number relative to your bonus. If you don't know the target number it's worth a little less than average. I usually assume +3 or so.
In cases where fumbles or critical hit/success is possible the reroll's worth a bit more.
In the specific case of bit of luck vs. touch of good, either's of dubious usefulness by level 8 - in combat you likely have better uses for your standard action by that point, and out of combat most actions to which something like this might apply take longer than one round.
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The easy way to do the math is to make a spreadsheet. Put numbers 1-20 vertically in A2:A21, put numbers 1-20 horizontally in B1:U1, and then take the max of the two. This shows you that the average Bit of Luck bonus is +3.33 (average roll is 13.83 with Bit of Luck and 10.5 without it.)
It's worth noting that the distributions are also different. With Bit of Luck, you're almost twice as likely to roll a nat 20, and your chance of rolling a nat 1 is 20x lower. Instead of a flat distribution where each roll is equally likely, it's a staircase distribution: Chance of rolling a 1 is (1/400), 2 is (3/400), 3 is (5/400), 4 is (7/400), ..., 19 is (37/400), 20 is (39/400). So for getting crits Bit of Luck is far-and-away better than Touch of Good.