if you want to go long long range then you want more then a 6mm ppc there the king of bench rest out to 300 yards maybe bit more. after that the br is better.
look on the 6mm br site thats where you want to look for longrange shooting info.
308 will be the cheaper bet. massive range of bullets and rifles. depends on the money you have to spend and what you want.
as for tikka or winchester both top rifles. i have a winchester and like it alot