Legildur said:
LOL! I could count on one hand the number of times I've seen a monk being able to hit a creature with a natural 5 on a d20....... There is a good reason that 'Flurry of Blows' is colloquially known as 'Flurry of Misses' in our campaigns. But the maths looks okay and demonstrates a point.
Actually, Tetsubo has a valid point. I was unaware of the two attacks at 11th level or higher.
Code:
1 –2/–2 -2
2 –1/–1 -1
3 +0/+0 +0
4 +1/+1 +1
5 +2/+2 +2
6 +3/+3 +3
7 +4/+4 +4
8 +5/+5/+0 +5
9 +6/+6/+1 +6
10 +7/+7/+2 +7
11 +8/+8/+8/+3 +8/+8
12 +9/+9/+9/+4 +9/+9
13 +9/+9/+9/+4 +9/+9
14 +10/+10/+10/+5 +10/+10
15 +11/+11/+11/+6/+1 +11/+11
16 +12/+12/+12/+7/+2 +12/+12
17 +12/+12/+12/+7/+2 +12/+12
18 +13/+13/+13/+8/+3 +13/+13
19 +14/+14/+14/+9/+4 +14/+14
20 +15/+15/+15/+10/+5 +15/+15
At levels 1 to 7, both Decisive Strike and Flurry of Blows average the same damage.
At levels 8, 9, and 10, Flurry of Blows averages more damage.
At levels 11+, Decisive Strike averages more damage if you have two opponents. Flurry of Blows averages more damage if you have one opponent.
Since the two attacks from Decisive Strike has to target different opponents, it is much weaker against one opponent.
This means that although on the surface, Decisive Strike looks slightly better, in reality Flurry of Blows is considerably better.
At lower levels (10-), Flurry of Blows can be used against different opponents, especially if one opponent is taken out by one of the earlier attacks.
At higher levels, Flurry of Blows averages slightly less damage if you have two (or more) opponents, but it is damage that can be targeted against one foe. Also, the damage from Decisive Strike can be "too overwhelming". In other words, if your opponent has 3 hit points left, Decisive Strike is overkill.
All in all, Flurry of Blows is better than Decisive Strike. Not because it averages more damage at the higher levels (which it may not), but because it is more versatile. And, if you only have one opponent at higher levels, Flurry of Blows will in fact average more damage.
Note: Damage Reduction changes this picture. There, Decisive Strike gains an edge in average damage, possibly even against a single opponent.