Weapon multiplier and %ATT are irrelevant when comparing 8 att vs. 5 att 5 all. They are individual multiplicative components of the damage formula, and are independent of flat att and stat boosts. However, %ATT can be used to calculate your total ATT if you don't feel like adding them up manually. It's the same idea with %stats being used to calculate your total stats.
If you want to be exact, I'd compare these two values:
Scenario 1, 8 att: (ATT+10*8)*(4*((bDEX*(1+%MW)+pDEX)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR)*%STR+npSTR))
Scenario 2, 5 att 5 all: (ATT+10*5)*(4*((bDEX*(1+%MW)+pDEX+50)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR+50)*%STR+npSTR))
where the prefixes are: b = base stat, p = stat affected by potential, np = stat not affected by potential. ATT, pDEX, and pSTR should be the values with your clean AB weapon equipped.
And see which scenario gives the higher value. Of course, this assumes a +10 scrolled, unenhanced weapon. Any subsequent enhancements will most likely point toward the conclusion that 8 att is superior to 5 att 5 all, anyway.
If you want to be exact, I'd compare these two values:
Scenario 1, 8 att: (ATT+10*8)*(4*((bDEX*(1+%MW)+pDEX)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR)*%STR+npSTR))
Scenario 2, 5 att 5 all: (ATT+10*5)*(4*((bDEX*(1+%MW)+pDEX+50)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR+50)*%STR+npSTR))
where the prefixes are: b = base stat, p = stat affected by potential, np = stat not affected by potential. ATT, pDEX, and pSTR should be the values with your clean AB weapon equipped.
And see which scenario gives the higher value. Of course, this assumes a +10 scrolled, unenhanced weapon. Any subsequent enhancements will most likely point toward the conclusion that 8 att is superior to 5 att 5 all, anyway.