Incarnate is much more flexible. In addition to the mentioned Opportunist buff, there's also the ability to make the Incarnate run any type of damage, and qute a few defensive immunities. Need Fire damage, get a Fire surface and summon him onto that (and he gains fire Immunity) etc.
Additionally Incarnate has a better to hit %, I noticed on the Spider it was significantly lower (don't recall exact numbers at the moment).
Unfortunately, (although probably reasonable from a balance perspective) all the 'big' summons occupy the same slot, Incarnate = Bone Widow = Artillery Plant = Ifan Soul Wolf... summoning one kills the old.
