The barrel in the 280 AI will last and last. The 6.5-284 will not last as long if that makes a difference since you plan on shooting targets quite a bit. Either will kill your deer but if you plan on shooting targets the longer barrel life would be important. The 280 AI is far more versatile a cartridge for future use with a wide array of bullet choices for hunting and targets.
The 308 will last forever and is cheap and easy to reload. Slow with a lot of clicks for windage and drop. In a hunting situation to 700 yards it would be far more difficult to make hits in the wind with the 308 vs the 280 AI.