Hey Hungry I new some one out there had to have asked this question before. This is what I found.
1000 yard rifle is a 6.5-08. 26" barrel with H4350 can move a 142 grain Sierra Matchking close to 2800fps. This is a fine 1000 yard round, however the 6.5-284 will launch the same bullet around 200 fps faster. Not enough to run out and buy a 6.5-284 if you already have a 260 Remington (6.5-08), but it is a significant difference in my opinion.
You should get longer barrel life with the .260 over the 6.5-284 in most cases.
The big reason for getting away from the 260 is brass. For a casual shooter, the 260 works fine but for competition use the brass is just too much of a problem to get right. The easiest way to get good stuff seems to be to buy Lapua .243 brass and run it thru a neck expander. I tried sorting and utilizing Remington brass and it is just too much trouble to get good stuff. For somebody who shoots open sights and shoots around 90%, it is probably usable, but for F-class where 75% of the shots need to end up in the X-ring and anything below a 10 can lose the match the 260 Remington brass just isn't up to par.
The Swede and the 6.5-284 both have Lapua brass available, so it really comes down to preference. Both have the same case length but the Lapua Swede brass has a larger rim diameter. My Swede rifle was originally a Remington 243 Varmint rifle. The Swede brass is a bit of a tight fit in the bolt face but works as long as I am willing to pick out the empties by hand.
Enter the 6.5-284, great brass, correct bolt face, works like a charm. Don't get too worried about burning up barrels in a hurry. Simply load the round down a bit and barrel life goes way up. You don't have to rocket along at 3000 fps every time you pull the trigger but it is available when you want to try it out.
would go with the 6.5 x 284. It's a tried and true 1000 yd cartridge, and premium factory brass is available from Lapua and Norma (or you can neck down Winchester 284 brass if you want) and reloading dies are readily available.
As to barrel life, I think you'll be pushing a 260 Rem to flat out max loadings and not get close to the 6.5 x 284 performance (especially in the wind). When you push the 260 Rem that hard, your barrel will go a lot faster than you might think anyway. Same with the 6XC, start pushing Sierra 107's or the Tubb 115's up in the 2950 to 3000 fps, and your barrel will erode quickly too.
There's no short cut here, if you go for barrel life, you are going to compromise the performance of the rifle, and if your like most of us, a day at 1000 yd range is a special day that does not happen regularly, so treat yourself and go with the best you can.
Two people you might want to talk to are Terry Cross of KMW (he shoots the 139 scenars) or Greg Langlieus both of them have extensive experience with this round.
Just for fun have you looked at the 243 with the new 6mm bullets in 107, 115 and soon the 120 it may be an interesting choice.