I will offer my vote for the 6,5x55.
At the ranges described, the 6,5x284 has only a slight advantage in killspace and winddrift.
Roughly speaking, with 600m the 6,5x55 has a killspace of about 585-610m where the bullet strikes 1 fist above or below the crosshairs. The 6,5x284 offers about <10m more. With a rangefinder the trajectory is a non-issue as you'll easily be able to fit your target inside a killing window. Energy is ample for deer.
The 6,5x284 truly is a better longrange cartridge than the 6,5x55, but the benefits of the latter will possibly outweigh the former in your scenario. Match shooters at national level here routinely/indiscriminately switch barrels at 6,000 rounds. I got almost 9.000 rounds worth of hunting accuracy out of my last barrel. Spread was about 1,5"@100m when I scrapped it. Very easy to reload, "everything" seems to shoot well when using some of the quality components available.
You can expect to get 820 m/sec (2650-2700fps) with a 140gr and 880-900 m/sec (2900-2950 fps) with a 120gr (24" barrel). Take a look in a ballistics program and see how the super 6,5mm bullets holds their own passing 500m before dismissing these seemingly unimpressive muzzle velocities.
Good luck!