What to re-roll on weapon in order to optimize DPS?

The formula for DPS of a weapon is

(MaxDamage+MinDamage)/2 * AttacksPerSecond * (1+Damage%/100)

If you upgrade your +damage stat to +1290-1490, you will have an increase of 143 of both Min and Max Damage, so you end up with

(1458+1882)/2 * 1.4 * 1 = 2338.0.

OTOH, with +10% damage, you get

(1315+1739)/2 * 1.4 * 1.1 = 2351.6,

which is only slightly higher. Note that in the first case you keep the 9% less resource cost, which can translate into a bigger increase in actual DPS than the nominal 13.6 DPS.

Here you can calculate your weapon damage pretty easy.

If you re-roll resource cost reduction to +10% damage your weapon DPS will be:

2375.3

If you re-roll +damage affix and keep resource cost reduction your weapon DPS will be around:

2338.0

(I calculated with +1290 - 1490 Damage; 1490 Damage is max and 1290 will vary so the result will vary too)

Conclusion:

For really max damage you should re-roll resource cost reduction to +10% damage. But if you only give up around 50 DPS you can keep 9% resource cost reduction which is nice too. I would say it depends on your skill build. If you need resource cost reduction you should keep it. If you dont need it, re-roll it to +10% damage. Personally I probably would keep resource cost reduction and re-roll +damage affix.

From this quite useful guide (in French, sorry), the priority order of a weapon's stats is:

gem slot > main carac. > % damage OR % attack speed > defensive affixes Why the gem slot first ? Because you CANNOT live without the 115-130 Critical Hit Damage that an emerald gem will provide.