I would take seatguru's advice with scepticism, it's gaining a bit of a reputation as being quite out of date. Case in point - the lack of in-seat power you mention is completely wrong - see the guide to in-seat power here - every VS seat in every class has at least USB power.

As far as a seat-for-seat comparison, if you're willing to wait a couple of weeks I am doing LHR-BOS-LHR, outbound in Economy Delight returning in Comfort+ so will be able to give you my take on things.

At the moment VS are using three different aircraft types for NY-LON, A333, A346 and B789. DL at the moment are using A332/A333. If I was looking at VS Dreamliner at 3-3-3 in Economy vs. DL A330 at 2-4-2 I would be leaning towards Delta. But this far out there is no guarantee which aircraft you'd be using.

If you are in a hurry to book, I'd stick with the VS daytime flight out to London for the EU protection (you will get none on Delta), and take Delta going back. Just so you can say you've tried both. :-)