Lightning:

Con:
Deals lowest damage out of all casting charms.

Pro:
Deals highest damage out of all casting charms.
Cheapest charm to purchase. [Unless this has changed]

Constant:
Guardian can not use any attack casting charms.

Note: As stated previously, build your set on your best damage charm level 45-55 and work backwards to make a full set. Also, fill in the missing charms with best damage output until you can get the damage type you want at little cost... remember, your character screen will show you the best results when you have those charms placed in your inventory no matter what your character level is based on its' intelligence etc.