I was in the same boat recently. I wanted a 6.5-280 Ackley with a 26" carbon barrel. I had to order the barrel and it was going to take 12 - 14 months for the builder to receive it. I went ahead and ordered the barrel, reamer and started piling up components. Then one day the builder calls and says we have a 7mm carbon that will finish at 26" that the original customer backed out on, and if I still wanted a 7mm-08 carbon they would make me a deal. I had already ordered and paid for a 24" heavy contour 7mm-08 rifle that was going to be done in a week or two, so I decided a 280 Ackley was the logical step since I had brass, primers and Whidden bushing dies for the 6.5-280 that could easily be used for the straight 280, the rest was history. They chambered the barrel and spun everything together and about 2 weeks after I picked up the 7mm-08 I picked up the 280 Ackley. It quickly became my go to, accurate and easy on the shoulder, easy to carry at 8 3/4lbs dressed out and with ammo. There is also the benefit of not having to resize brass or fight for bullets with the 6.5mm manbun crowd.
I vote 280 Ackley for now, but keep the components build the 6.5-284 down the road to use for whatever you deem necessary. This is exactly what I am doing with the 6.5-280 Ackley, except that I am trying to get it as light as possible.