Well you are discussing the difference between 6.5-284 and the .25wssm. You are not discussing the same things if you want the better of the two, especially if you are considering the .25 on the AR platform. To be honest, I don't have personal experience with the 6.5-284, but I do know several guys who have them and swear by them for hunting and target shooting purposes. I do have an Olympic upper chambered for the .25 wssm. First off, if you don't reload or do not intend to reload, don't go this route. Factory loaded ammo currently available is mostly too long to be used in the AR platform. You are restricted to the COL that will fit in the AR magazine and this will not allow you to use the heavier bullets of the quarterbore's arsenal. Anything over 100 gr bullets must be seated deeper in the case which makes you lose powder capacity, which in turn reduces velocity. You can achieve excellent accuracy and good velocity, but not as high as with a bolt action variant for this cartridge. You are mostly limited to 100 or 110gr bullets in the AR platform for the .25 WSSM. You can load larger, but with reduced velocity.
Also, you are not being equal with your consideration of bullets. In my opinion you will get better long range stability from the 6.5mm (.264) bullet than the .257 bullet. I shoot, and love, my .260 Remington, but I also like my .25 caliber cartridges as well. The 6.5-284 would be similar to a .260 Magnum, with the true meaning of magnum, like the difference between .22lr and .22 mag. The .25 wssm is good for lighter ranged bullets, the largest your can really get is 120gr and is not suitable for the AR platform. The 6.5mm is available all the way up to 160gr pills, but are not really necessary for N. American game.
If you want the new and cool AR then the .25 WSSM or the newest .30 OSSM would be your ticket. If you want a good bolt gun then I would say you would be hard pressed to beat a 6.5-284. Of course there are numerous others that are equal in power and versatility, but if you want an odd named cartridge this would be a good one. Neither cartridge is going to be cheap to by factory ammo for neither will either be easily found at the local Wal-Mart in the future. Honestly, I would look at either of these two cartridges as a reloading endeavor, with the availability of brass for either to be the same. Both can be made from other cartridges, but WSSM brass is much harder to work, as I can attest.
If I hadn't gotten bit by the .50 Beowulf bug several years ago I would never have gotten into AR's because I didn't have a use for .223. But since then I have found the black gun to be quite intriguing and fun, although a different beast entirely than a bolt gun, especially with these new aged uppers with larger cartridges.
With equal bullet weights between the 6.5-284 on a bolt gun and the .257 wssm on the AR platform, I would say your velocities would most likely be higher with the larger diameter 6.5 simply because of the case volume and the ability to achieve higher pressured loads on the bolt action. With the AR you are limited to COL because of the magazine and you have to be careful not to over pressurize the gas system.
In closing, I will say that I do believe that the WSSM cartridge in any caliber is the optimum platform for a big game hunting AR. They are fast, sexy, and will definately get the job done; within reason. I think the AR is what will save the wssm cartridges since Winchester dropped them from their production line. I doubt if Browning will continue them much longer either.