They both get 3 types of pet, fighter, scout, and mage. Conjurers get the good version, Necromancers get the evil versions.
For Necro, you get these pets at these levels:
1 - Summon Minion (Bat Warrior)
12 - Summon Servant (Spider Warrior)
20 - Grim Spellbinder (Zombie Warlock)
24 - Undying Adherant (Skeleton Warior)
32 - Shadowy Stalker (Shadow Aassasin)
34 - Grim Thulian (Skeleton Warlock)
38 - Rotting Thrall (Zombie Warrior)
46 - Shadowy Assassin (Shadow Assassin)
48 - Grim Terror (Succubus Warlock)
For Conjurer, you get these pets at these levels:
1 - Tellurian Recruit (Pet: Beetle Warrior)
12 - Tellurian Follower (Pet: Earthcrawler Warrior)
20 - Igneous Apprentice (Pet: Fire Elemental Mage)
24 - Tellurian Soldier (Pet: Earth Fighter)
32 - Aery Outrider (Pet: Air Scout)
34 - Igneous Adept (Pet: Fire Mage)
38 - Tellurian Avenger (Pet: Earth Snake Fighter)
46 - Aery Stalker (Pet: Air Scout)
48 - Igneous Magi (Pet: Fire Elemental Mage)
Necro's do poison/disease damage, are generally better at solo encounters, and have life-siphons to heal the pet. Conjurers do elemental/magic damage, are generally better at against groups, and have better pet procs.
Both are very effective at soloing and groups, as long as it's played smart.