The 278 is certainly a great unit, it has most of the stuff that I would really like to add to my 276C (i.e. all of Europe preloaded and more features with POIs meaning speed cameras can be added) but at the moment as I have a 276C already it's just too expensive to justify.
I completely agree with your usage summary though, particularly with regard to the screen, the quality of visible maps is a decent bit better than with the Zumo and although the user interface is rather dated, the power of the tracking and route features in particular over the Zumo make it interesting for me.
For the problem with the screen, I simply bought a cover from
http://www.shieldzone.com/form/index.html and the problem is solved, the clarity reduction is minimal and I've used it for about 4 months and still no screen scratches!
(the only downside is that in strong sunlight, the guard expands more than the screen and can start to peel at the corners, but it readily sticks back on and this has only happened once)
The Sirf II antenna is the biggest disadvantage of this unit as it means that in towns and trees that the satellite reception is more likely to be lost than with the Zumo (it's also a lot more difficult to get a reception indoors) but as this is primarily a marine unit, I guess there aren't such problems when you're sailing (or at least if you're so close to trees or buildings in a boat, you've probably got more pressing issues at hand than losing satellites!)
Thanks for posting the info
David