I go with the damage multiplier
I tend to favor the damage modifier more then the threat range. Scoring a critical at x2 is nice, but it really comes down to being the same as scoring two hits in a row with the weapon. Also, weapons with x2 modifiers tend to be used in one hand, which means that the str damage is lower. An 18 Str crit with a longsword amounts to 2d8+4 (before counting weapon enchantment, specialization, etc).
However, a x3 modifier is just more impressive, especially when you consider that many of these weapons tend to be used two handed (Great Axe, spears, most pole arms). The x4 modifiers are even better, but the dice range is not as good (which is important since you cannot hit a critical all the time).
A Great Axe used by an 18 str char will do 1d12+6. A critical with that weapon will do 3d12+18. The same character using a Great Sword may do more damage over time (2d6+6 normal, 4d6+12 crit), but the criticals are just not as awe inspiring.
Another thing to consider is where you think you will be getting most of your damage bonuses from. The magical enchantments, specialization bonuses, and the like stay the same for one handed or two handed use. However, Str bonuses to damage on two handed weapons are massively magnified by a high critical modifier.
Great Sword, 2d6 (x2 multiplier)
14 Str 2d6+3, 4d6+6
16 Str 2d6+4, 4d6+8
18 Str 2d6+6, 4d6+12
20 Str 2d6+7, 4d6+14
22 Str 2d6+9, 4d6+18
Great Axe (x3 multiplier)
14 Str 1d12+3, 3d12+9
16 Str 1d12+4, 3d12+12
18 Str 1d12+6, 3d12+18
20 Str 1d12+7, 3d12+21
22 Str 1d12+9, 3d12+27
Scythe (x4 multiplier)
14 Str 2d4+3, 8d4+12
16 Str 2d4+4, 8d4+16
18 Str 2d4+6, 8d4+24
20 Str 2d4+7, 8d4+28
22 Str 2d4+9, 8d4+36
The numbers speak for them selves. A 4 point increase in Str is not too hard to obtain (Spell enhancement, magical items, barbarian rage). While you will certainly see the Great Sword critical more often then the Great Axe or Scythe, the damage difference on a critical is staggering when a high str is factored in. Lets take a quick look at the min and max damage for each critical assuming 18 Str.
Great Sword: 16 to 36 damage.
Great Axe: 21 to 54 damage
Scythe: 32 to 56 damage
A critical from a Greatsword will shorten a fight, and with Improved Critical, they will happen fairly often. But the flipside is that your opponent will take a bit longer to die. A Critical with a Great Axe will drop an opponent if you have been beating on him for a few rounds. A Scythe critical's minimum damage is within 4 of the greatswords maximum! Consider your self lucky that no x4 weapons do better then 2d4 (Though a Giant weilding a huge scythe....).
If you have a lower Str score, then going for the threat modifier is probably a safer bet. But if you have the Strength of a minor diety, go for the critical modifer. A x2, Threat 19 critical makes for a good average damage. A x3, Threat 20 critical does not do much for the average, but it can very suddenly end a fight.
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