Hey John,
this is an interesting statement: "Once you have decided what you are doing, the choice is normally pretty clear."
what areas do you think play to the strengths of each machine...?
OK ............................
Really the differences between the 788T and the Nagra VI.
For pure audio quality I would say that the Nagra is the better (which is why it is being used by quite a few classical music location recordists), it can run for 12-15 hours on the high capacity battery pack and runs cool. It is not heavy, but a bit large for over-the -shoulder stuff and probably better suited to a trolly or table-top use (which is how I use mine).
The 788T is about half the size, not quite as good audio-wise (but still better than just about anything else), but runs hot. It is much better suited to over-the -shoulder stuff (but you need to leave air-space in the "handbag" - uniquely it will also take AES42 mics (I need the Neumann DMI-2P do do this with the Nagra). The Nagra VI IMHO has the better ergonomics and the menu structure is certainly much better.
Both machines average out at about the same price, I think - The Nagra is cheaper in the UK and the SD is cheaper in the USA - so they are probably the same on average. You get 8 mic. pre-amps with the SD and only 4 with the Nagra but at a higher spec. - the differences are not large, though. The Menu structure is more awkward, though you can set short-cuts and the new iPhone app may get around a lot of this.
So, if you want the ultimate quality in music recording, the Nagra comes out top - if you are doing location recording for broadcast the SD comes out top.
Anyway, this is my take on it.
I like both machines.