Personally, I prefer conjuration. Here are my reasons:
* Chain casting -- summoning pets is conj.
* Healing -- healing is conj.
* Magic DD -- our best magic based DD is conj.
* Fire DD -- This is where many people part. Personally, I use the lvl 70 Ancient spell Nova Strike which is conjuration. I prefer it over bolts for a few reasons.
1) It casts faster and is good for when I need to toss heals in. If I'm needing to heal pet then I definately don't have time for my bolt spell to cast.
2) It draws much less agro. Technically, it probably draws the same agro per hp damage. However, if you have agro reducing AAs then that agro is spread out pretty evenly where the bolts would come in spikes. During raids I'm lucky to empty half my mana tank before drawing agro with bolts. However, I can get down to 25 percent, sometimes an empty tank using DDs. And my dps is really not that much lower if at all.
3) Specialization aside, I find the Nova Strike just as mana efficient if not more so. IMO bolts are only more efficient on paper. In practice they are no more efficient than the DD. I seem to get fewer resists with the Nova Strike. Also, if I'm raiding and some wizard kills the mob before my cast finishes then I'm not charged the mana; however, with bolts I do get charged unless I'm not on mount and can duck in time. There are many other similar things like bolts already in the air when mob dies, bolts getting hung up on tracking or something, etc. Don't get me wrong, there are a few encounters I can think of where bolts rock. And for those fights it's still an option for me. I just loose a hair more mana is all.
Bottom line, if you feel you have a shot at getting an ancient rune then I think it leans in the favor or conjuration. However, if you feel you won't see an ancient rune then I feel it's a coin toss.