Question: Is there any ranking as to the usefulness or rarity of some of the offered quest rewards? Yes, I know that its possible to just run through the content multiple times and get all of them - but I don't want to do that yet, my furthest char is Act 2 Normal ;)

What considerations should I think of, for e.g. when choosing between `Added Lightning Damage` and `Weapon Elemental Damage` for a Templar? Are there general 'rules of thumb' to keep in mind?

Answer: 
If you get offered 2 or more gems that you actually need for your character then pick the one that's harder to find and more expensive to buy from other players. Of course you should also check which gems you'll be getting from quests in the near future in case one of those gems will appear as reward again. [Check them here.](http://en.pathofexilewiki.com/wiki/Quest_Rewards)

If you don't get any gems you need for your character then you can:

* Pick the one you might need for your other character, if you have one or plan on making one
* Pick the one that sells for the most to other players

In case you get bad gems as reward which you don't need, and other people don't want to buy then just pick either one and delete it

Do note that there aren't that many quest reward gems that are actually worth something. Since they can be obtained from quests their prices are low to nonexistent.
For reference to anyone coming to this question post-2.0.0 update of the game, most, if not all, gems that are quest rewards can subsequently be bought from vendors.