Well - it's been a difficult choice process, and I've been following threads on the Sigma, the Tamron, and long macros.
To cover what I need now, I've finally gone for the Tamron 70-300 and a Sigma 180 Macro prime. The Tammy will do me until the next DA* lens arrives (with either 250 or 300 at the long end) after which I'll sell the Tammy. The Sigma 180 Macro will be for shooting very skittish water bugs.
Thanks to all who have posted on the various lenses, especially Metalfab who has gone to a lot of trouble - very much appreciated. In the end, the Sigma 70-300 lost out by half a fartskin, mainly on price and sharpness at the longest end, particularly after reading the reviews. But it was a close thing. I can live with some purple fringing if my copy of the Tammy proves to have it, as it is easy to remove in PP. But there seems to be quite a bit of sample variation - those pictures put up by Jay Sherman are very impressive, and PF seems not to be a problem with his Tammy 70-300. I wonder if they've modified the coatings on the newest copies perhaps, as Jay just got his very recently?
So, thanks again troops. What a great resource of information and informed opinions you lot are
This forum has been a great find
Orders for the Tamron 70-300 and the Sigma 180mm Macro are in, and I should receive both in the next week or so.