overall, i think incarnate is much, much more useful. he can do all kinds of magical damage, can CC with a battering ram, has a ranged attack (unlike the widow), and has a lot of armor with infusions. widow's huge HP doesn't solve the armor problem, since she'll just spend all her lifetime incapacitated. she's still useful if you need a quick damage burst or a teleport though.
as a side note, summons don't just soak aggro anymore, since the AI will always ruthlessly target your most vulnerable character, and it will be the summon only if your PCs are really tanky or inaccessible (or if it's the widow, due to her low armor).
P.S. one useful trick is to enter fights with a pre-cast summon, this way you can re-cast it without a cooldown.
