Hunting both elk and varmints with the same rifle makes for a very hard choice.You will either choose a smaller cartridge to reduce recoil for varmint shooting and be on the small side for elk or choose a cartridge suitable for elk and have a poor varmint cartridge.I would rather be overgunned than undergunned so I would choose a good elk cartridge and put up with more recoil in a varmint gun.That being said of the choices you give I would choose the 280rem as it shoots a slightly bigger bullet than the 270 for elk and shoots flatter than the 30-06.Although you don't mention them I would also consider the 7mm wsm and 7mm rem mag as both are a little more powerful than the 280 and have less recoil than the 300 winmag.