What should I spend dark elixer on?

Should I spend my dark elixer on leveling heros, or leveling the dark troops in the lab?

Depends on the level of your hero's at the moment i guess. Are they atleast level 5 for the abilities?

Get ur heros leveled up to a decent amount and than go into the labrotory and start upgrading troops (Dont upgrade the valkyry, they suck)

It depends on what you are using the most. All troops sucks at level 1. So I suggest upgrading minions to level 2 at least. Your king should be level 5 at least. After that it's a question of choice. (queen is really expensive)

Minions : level 2 takes alot of hits to kill a king. level 4-5 will kill him much faster
Hogs : well, too late for that. Mine will be level 4 in 3 days. They will remain there for a long, long time.
valk : don't bother with that
Golem : at level 3 it begins to be good. Only if you have alot of DE
Witch : this is a good investment. For now you only need 1 upgrade and she is much stronger at level 2.

Good luck