Tahr at 400+m - really? Every time?
I have had a .25-06 for over 10 years and load my own, so have a good feel for what it can and can't do. For large deer I can think of probably 20 chamberings that are far more suitable at those ranges. Unless the barrel is at least 24", and ideally longer, I wouldn't recommend it beyond 250m.
Of the two you mention I'd pick the .270 at the drop of a hat.
If you are not set on one of the two, I'd pick neither, and .243 is a far from ideal deer chambering too.
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