18/2 is at 602.24 using the same rules as the chart I posted. The 18/1/1 is at 596.82 without SA so you're gaining just under 6 DPS for Fighter Str I.
So for the last level of Barb your choice is:
5.42 DPS from Strength
or
20.27 DPS from Sneak Attack
Against 50% fort you're still gaining more DPS from SA, however at 100% you gain no benefit from SA.
This of course also relies on you avoiding agro and as this build is the highest DPS build (without threat reduction) you wont be benefiting from it all of the time. The sneak attack will provide more damage against autocrit targets (6.5 average vs the 4 from str with picks). With the 18/2 you also get an extra feat and can include the barb past life or another feat to increase DPS (OTWF or other).