There is no 'better' between those two, they are just very different.
The ES is more versatile due to the usefulness of the merrow, but they're both strong classes - and the PSs fourth servitor is certainly a lot more fun to use than the seraphim.
My favorite is phantom summoner, but that's because I am a phantom summoner (soon to be spectral master, yay) :p
PS is an enjoyable class. They aren't as strong in AoE -- they have the same skills as the necro, corpse burst and poison cloud, but unlike the ES they can't nuke until there's a corpse to pop. Regardless, back when AoE groups actually seemed to exist, I did a lot of AoE as a PS. I'd just have my summon lead with a poison cloud, then hit a monster until dead, then pop corpse and repeat. I never did AoE solo though, always in AoE groups.
The PS lacks some of the versatility but it's still quite versatile. If you have a buffer in the party (I have a dualboxed warcryer so I can always have a buffer if wanted) a PS can be a very effective healer (though you CAN'T cure poison/bleed/para so it limits you in some ways). The Nightshade for the powerful, single target heals, and Chant of Life/Chant of Vampire keep up the others unless the AoE damage is way too high.
On raid bosses, the PS is an incredible tank healer. The reason being, we can't run out of mana. When I heal a tank in a RBG, I summon nightshade (of course) and spam Shade Sacrifice (HP transfer from shade to target) while spamming Servitor Heal (to regen my summon's HP). When my own MP gets to 1/3, I stop servitor healing entirely, and simply desummon and resummon Nightshade when he runs out of HP to transfer. I regenerate the MP from summoning Shade faster than I can spam away all its HP, so my MP never goes down in this phase. With acu, zerk, an acumen weapon, +cast speed robes, and bsps I can summon a new shade very fast so the interruption in healing is quite minor.
A PS won't replace a bishop but they make a very fine addition to the group.
ES have an AoE nuke they can cast at any time (plus can servitor-buff empower).
PS have an AoE nuke that can only be used if there is a nearby corpse, and they have servitor-might instead of servitor-empower.
Oh, another thing I like about the PS -- mass summon phantom cubic.
In a party of nine I can pretty much guarantee that virtually every single monster we fight is hexed, weakened, and windshackled.
Mass Aqua Cubic, because it's a DOT, really doesn't get the same benefit -- once one person lands it, the future firings of the other people's cubics don't really add much. Because the Phantom can debuff randomly one of three things, it gains more from repeated application.
Oh, another thing I like about the PS -- mass summon phantom cubic.
In a party of nine I can pretty much guarantee that virtually every single monster we fight is hexed, weakened, and windshackled.
This is actually my only real complaint about the Warlock. My son has a PS and another clannie is an ES. I can tell you without a doubt that the PS mass cubic is the best of them all in a large party. Why then does my cubic (Mass Storm) cost more than twice the D-crystals of the PS or the ES?
At level 58 my mass cubic costs 36 D-crystals> the PS and ES?......16 D-crystals
When we are in a large party, we always use the PS's mass cubic, not for the cost though. By using that cubic we kill so much faster and take so much less damage it isn't even funny!