Being you have a 243 the 308 would compliment it nicely. The 243 is a necked down 308. But Sheridan and emtrescue also recommended some good cartridges.
Do you handload or buy factory ammo? If it's factory ammo might want to consider what's availible in your area.

As far as brand of rifles, pick the one that feels best to you,buy the best that you can afford.You definately get what you pay for here.