I'm surprised it hasn't been mentioned (much) yet. One big factor here is what rate of twist the barrels are. You need the heavier bullets for shooting long range, so if one barrel has a slower rate of twist, it will probably stabilize the lighter bullets better, making it not as desireable for long range shooting.
Which one is more accurate? Mechanical accuracy trumps any other consideration.
I'll agree this right here is the single biggest factor in which one you should choose to setup for long range.
If they were equally accurate, I personally would go with some Berger VLDs in the .270. While I can handle the extra recoil, it's easier for me to shoot a gun with less recoil accurately.
Here is an interesting article on using the .270 for long range shooting.