so then..
I'll take that 4d6 (i.e. 14 average damage) and take it over 19 swings (since 1 is a miss), so that's 0.7368 average damage per hit. I'll input that as extra damage. And I'll put in a 15/x3 threat range (i.e. a barb with a khopesh) with 44 str and compare it to one with 46 str.
Sword and shield: blast is better than str (except at high AC)
http://www.afterlifeguild.org/Thott/...ac=10&maxac=70
(you can click modify to see input)
The benefit of str is greater for 2H, since it's 1.5x. Also used a maul here. +2 str wins
http://www.afterlifeguild.org/Thott/...ac=10&maxac=70
and then say for twf one is a blast and one is +2 str, VS both being blast:
http://www.afterlifeguild.org/Thott/...ac=10&maxac=70
(str edges out here since it helps both weapons)
If you want to compare good blast (4d6 for x3) instead of fire blast on a twf khopesh barb, then you get this:
http://www.afterlifeguild.org/Thott/...ac=10&maxac=70
(it's 14 avg damage instead of 11, so good blast is stronger than elemental blast, but still not as good as +2 str except just barely at the lowest AC range)
So we can see that the blast critical effect on both weapons is actually not as good as +2 str on one of them and blast on the other for a twf barb using the best craftable shroud crit weapon. For a non-barb or a barb using a weapon with a worse threat range/multiplier, then the +2 str on 1 weapon comes ahead even more.
Problem is, +2 str only comes on water or negative, and the tier 1 & tier 2 effects and final tier specials for those are not as good. Like you could go frost, icy burst (or +1 str), and +2 str, but then you get crushing wave. A mineral II or a radiance II or a lightning II would be better, although I guess for single shard water water water is OK, but you wont get the max DPS using +2 str because the associated upgrade paths arent as good.