Well, there are some factors i take into account when i go for rare resources:
- Rare resources in Age 1 / Age 2 should be collected with care as merchants cost gold and wood.
- The 3 best resources in my opinion for the first ages are Diamonds, Sugar and Gems.
* Diamonds give you +20 wealth which is extremely useful in the early ages, and +10% com limit which is quite good for later ages, when you can get up to +50 com limit over your opponent.
* Sugar reduces all food costs by 10%, and that accumulates all the way through from the early ages.
* Gems give you Knowledge and Wealth, and makes your borders stronger.
- Dye, Silk, Furs and Papyrus should be gotten as soon as possible after those 3, as they help a lot with investigation. 25% is quite a lot.
- Silver is slightly less useful, but still gives wealth which is useful.
- After that adapt to your rares, if you got Horses in your territory, you might consider HA-HC raiding, if you have Salt you may base your army more on barrack units. Of course if you have good food/wood, and are not in a water map, you might consider ommiting Citrus, if you have Statue of Liberty and good food/oil, you might ommit Titanium
- As Nubians you should always go for the rare resources in your territory, as they give you big bonuses.
A merchant is more or less the equivalent of 2 1/2 citizens, but with more Taxation techs (Up to 4), they can get to +50/+50 (max I have gotten out of titanium with non-nubians), AND you get the bonus.