To the O.P., I don't think you can go wrong with either. I've had a 264 since I was 11. It was my first rifle and to this day is still my favorite. That said I am a little bias toward the 264. I have nothing against the 270wby but already having the 264, I never could justify getting a 270. You will get different answers to your question. Some based on factual differences between the two but most based on the fact that the guy who answered has one or the other and is going to tell you the one he has is better than the other. In my opinion, with the new offerings for the 270, differences in ballistics between the two will be minimal within their effective ranges. The difference in frontal area is only .013". A difference that the animal will surely never notice. If you enjoyed the rifle in 264 form, stay with it. If you want something different, go 270. If you really want something different try a 6.5/270wby. I've been entertaining that idea for no other reason than to be the only guy at the range that has one! To each their own... Good luck with your decision. You'll be happy either way you go.
The shoulder angle acts like a funnel directing the hot gases into a flame path moving towards the throat and onwards. Where the two shoulders would meet in an imaginary angle is known to be the hottest point. The narrower the angle the further the vortex is projected out from the shoulders into the neck. Ideally we want this place to be in the first two thirds of the neck, or maybe even less. Now to get back to the .284 case a sec. The 35 degree shoulder angle is good, but the short neck length takes most of that advantage away. On the otherhand the 6.5-06 is just as bad with it's way too acute shoulder angle, but it does have a much longer neck length (about .32"). This is why a 6.5-06 imroved (there are several) has such a good barrel life. It's not really a knock on either case type, but it's to point "this is what you get." I personally like the 6.5x55 AI case design using a 6.5x57 as a parent case to make up for the known case shrinkage during fire forming (will be close to .04"). If you don't shoot but a hundred rounds a year, you'll never worry much about the issue. Yet if your a "F-Class" shooter you will.
The 6.5-06 is a little bit overbore as is the 6.5-.284. Not a tremendous amount, but still overbore. The 57mm case is not. Kinda reminds me of three guys I know that shot the 6-.284 in long range varmit rigs. They all used high end barrel blanks, but still changed barrels often. One of them gets the idea that he's going to mod the .284 case by pushing the shoulder back .100" with the rest going into the neck. He looses about 50fps, but is able to use the same barrel for well over twice the rounds. His buddy likes his idea and does one .150" short, but trims the necks back to about .33" (the other guy used the same neck length as a 6mm remington). He looses about 75 to 80fps compaired to the 6-284, but his groups are better. He gets almost two years out of the barrel! One of these guys sent me a pic of a marmet he shot at 800 yards using the Berger 88LD bullet. Had a grin on his face big enough to stuff a soft ball in it!!
You asked the following: "How can you say this, and keep it simple?" Easy actually. I know someone will go to great lengths to try and explain it & it doen't take long to read it!
You will not make peace with the Blue Coats, you are free to go.
If you guys are concerned about the short neck length, why not just lengthen it like I did?? I have a 264 Win with a .295 neck. The longer neck helps with seating, especially with longer VLD bullets! I shoot 140 grain bullets 3300 fps! Currently, I have over 2000 rounds thru the barrel and it is showing a little erosion. The last time I reloaded, I seated the bullet out .1 futher, and my groups have always been under 1/2. Instead of arguing back and forth about the short neck and the possible negative factors associated with it, just resize some 7mm Rem brass, use the longer neck and negate the possible negatives. On the plus side, it makes seating a whole lot easier!!
While I wish that Wby had a 6.5, I don't know why the .264 Win isn't more popular, other than it had the wrong barrel length and twist when it came out. With the slower burning powders, improved barrels, and better bullets of today there is more reasons to shoot the .264 Win.
My .264 Win is my favorite to shoot, and it has never let me down!!