The big thing I'd missed is that there are some Nikons close to the specification of the 500D that do support the older autofocus - the D90, for example - and aren't hideously expensive; I can get a D90 body for about £20 more than the 500D. This leaves me with three pretty good lenses plus extension tubes etc. and no urgent need to upgrade anything else. The big one I don't have at the moment is a good 50mm, I've played with a 35mm as my standard lens and it doesn't really work for me.
So for Nikon the upgrade path would be to get a D90, then sell my current D50 and look for a good 50mm, then sell off my Nikon 35mm lens and probably the Canon 50mm lens. I'd probably come out well ahead on the lenses overall.
For Canon the path would be buy the body, then look at upgrading at least my main zoom to a good macro model - say £100+ after trading in the existing zoom. I'd recoup most of that by trading in equivalent Nikon gear and would probably break even.
There isn't a lot in it money-wise, but in terms of hassle the Nikon route is probably easier. I'm going to think on it for a few days, and make a decision next week.
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