I'm from Brisbane, Australia and I have just joined this website.
I have read most (but not all) the posts under this topic. I don't know much about the 6.5 x 55 but it is a specialised calibre here, mainly among militalty rifle clubs. I have no doubt it is a fine cartridge. But I would prefer the 270Win, probably the finest hunting cartridge ever invented for medium size big game. It was a person favourite of the late Jack O'Connor long time gun writer and trophy hunter (Weatherby Big game award winner and multiple grand slam holder as well). Jack could never sing its praises too often or too load. Now this guy has probably done more hunting of big game than 99% of the world's hunters and that carries a lot of weight. He preferred the 270 for a light mountain rifle (particularly after sheep) and he has taken all manner of African plains game with it as well. Mind you he didn't see one bit of difference (and his comments relate equally thereto) between the 270 and the 280 and the like. You might like to refer to "The Hunting Rifle" published by Winchester Press written by Jack O'Connor.
On a peersonal note I had a 270 on a modified mauser 98 many years ago and sold it (can't remember why) and I have regretted the day I did sell it because it is just sensational. I am rectifying that with the import from the US of a Winchester pre 64 M70 fwt in 270 Win. This will fill the gap in my rifle cabinet that was left when I sold my 270 more than 20 years ago. Since then I have been using as my all round rifle a pre 64 M70 fwt in 308 Win another fine calibre and for your needs perhaps a worthy contender. I did see a couple of posts to this effect. it is also a great cartridge and with a good variety of bullets it is very effective. The 270, however, will be the flatter shooting generaly, if this is an important criteria.
I should conclude with the final thought, that Jack O'Connon, when asked if he could chose only one rifle, had not reservation in saying a 30-06.
Good hunting,
Magnum308