This does leave the final question as per thread title - is it worth £50+ extra just to have a barimetric altimeter and electronic compass?
Simply put, no, not £50.
The geek in me would kind of like more altitude info but I gather the GPS still does this to a reasonable accuracy.
I'm not entirely sure the barometric altimeter makes it more accurate over long distance rides.
Jo has more stuff on his page:
Accuracy of Elevation Measurement Using GPSIt's all to do with the way GPSes determine altitude (which is derived from the position information and the model of the oblate spheroid that is Earth). Google for 'GPS geode accuracy' for more...
I've certainly got some tracklogs of my commute that, for some reason, puts London at 200m ASL, despite having 7 miles to sort itself out. (This is an Edge 705 with barometric altimeter.)
It's much better when it calibrates itself against a known position/height. The unit only does this some times outside my house, I need to investigate how to trigger it so that it does it all of the time.
In practice, the tracklogs of rides I've done using a basic yellow Etrex (no altimeter) and my Edge 705 (with barometric altimeter) don't look that much different at all. Neither seems consistently better than the other.
However, not having the altimeter means you don't get the elevation profile pages, which can be useful if you've uploaded a tracklog with elevations in it as you can see what is coming (I think).
I've never found a use for an electronic compass (none of my GPSes have one). If you're on a road you just need to look at the map page to work out which way is North, if it's tricky then start moving and it'll become obvious.