The .284 is a slightly larger case but they shoot very close to each other. The .284 chambering is quite common in custom short actions but it's off spring the 6MM -284 and .25-284 and 6.5-284 are all more popular than the parent round.

For the actual difference in performance I'd choose the 7-08. The gains in availability of cases and ammo totally outweighs any very small difference in velocity.

For flat shooting the 7-08 rivals the old .270 for tradjectory and that's impressive.