I've played a defiler to level 66, so can only give you some information about defiler.
1. Ward blocks all damage. All types of melee and magic(dot, dd, aoe) damage have to wear down the ward first before the person with ward on actually takes any damage. You can look at it as having temporaily hp boost.
2. Defiler soloing is slow. You only have 3 single target dot and 1 ae dot to work with from 1-59. At 65, you get another ae dot that can do great damage. In order to solo well, you will need to get the dog pet through AP and invest quite a few AP into pet proc.
3. Defiler get power regen at level 62.
4. Defiler buffs the most raw hp after templar.
5. Defiler debuffs dps, stats and attack speed.
6. The highest single target heal (long cast time and long recast timer).