I'd vote on ES.
Maybe PS got better nukes and whatsoever, but unicorns can AoE pretty well, and I've seen later on Unicorn Magnus(es?) 1-2 hitting Drakos, one hitting PI mobs and so on. Therefore it seems easier to level, also parties don't benefit as much from PS as the other 2 summoners (as in ES could join a mage party in DV without doing much, Warlock even better - they can join archer and melee parties as well).
Though dark elven racial skills sound better to me than elven ones, but they don't make that much of difference, ES seems to level faster, therefore I'd chose it.
Both of them can AoE, the difference is: EM has empower for summon, SM no, you need an improved kooka at least. And you can cast tidal wave any time, corpse kaboom needs a corpse. But both aoe skills, tidal wave and corpse kaboom have the same power.
PS has best cubics, a tank summon(nightshade and its curse that decreases pdef and stack with hex).
But in the end they will be the same: wynn summoner. I like PS because I like dark elf style character. but make a Es if you like elfs.
I voted for Phantom Summoners, mainly because of their Phantom Cubic that's a blessing for melee parties.
Give a bit of love to DE summoners :P
This cubic is quite awesome indeed. Sadly I can only get it to great use in kama/laba instances. Either way I quit my PS as I got bored of it. I rarely see other PS in ToI or anything to join melee/archer parties (cubic works on bow hits no?). :l