I bought a StreetPilot III fairly recently and I deliberated for a long time over whether to buy the European version for just over £1k, or the US version for about £550.
So far as I can tell, the only differences are basemap and the Euro version being multi-lingual.
With the 128MB rewriteable datacard that comes with the Deluxe version, you can load in pretty much the whole of the UK down to streetlevel from the Euro City Navigator CD. For trips to Europe, you'd have to reload the data according to your destination. It would only be if you went off those tiles that you had loaded in that you'd miss having the Euro basemap.
Buying a US version, you wouldn't get the Euro City Navigator CDs (you'd get the North American version - which I am trying to borrow right now!), but there are enough folk about with the Euro version to load up your datacard as required.
Best
Greg Masters