Google's official stance is that they are "roughly equivalent" and recommends to do what is technically simpler to implement (source: https://www.youtube.com/watch?v=_MswMYk05tk).
With that said, I'd recommend a directory over a subdomain.
Doing this consolidates signals to a single domain, which should then theoretically build more authority for all pages off of that single domain. This consolidation of authority results in rank increases, which have been documented here: http://moz.com/community/q/moz-s-official-stance-on-subdomain-vs-subfolder-does-it-need-updating. A subdomain would split signals from the blog and the rest of the root domain content.
So while Google "says" they're roughly equivalent, SEOs have seen tangible evidence that sticking to a single domain can be beneficial.
If you're able to go with www.iconery.com/editorial/, I'd choose that.
Hope this helps!