There's not a lot of difference. Both are necked down 30-06 cases, with .473" bolt faces. The difference in bullet diameter is .007". The 280 has a little less body taper and the shoulder is farther forward than the 270. This prevents accidental chambering ofa 280 cartridgein a 270 chamber. It also gives the 280 a slightly larger powder capacity.

I have one of each and prefer the 280. If you're shooting factory ammunition, the 270 is probably the best choice since you can find a wide variety of ammunition almost anywhere. 280 ammo is not so common in stores. Ammo manufacturers don't offer the variety of bullet type and weights in 280 like they do for the 270