I think the 270 wsm is a Awesome round, super flat shooting to 500 yards with a 130 gr. The 6.8W has a case capacity of about 3-4 grs less then the 270 wsm. Yes the 6.8W has a faster twist but shooting the heavies only outperforms the 270 wsm passed about 650 yards but not as flat before. Both the 6.8W and the 300 wsm have cases on the smaller side compared to others in their categories. Imo the 6.8W was a knee jerk response to the noise the 6.5 prc was making. Its ok. The 300 wsm is the smallest 30 cal mag and set a record group size at 1000 yards resently at just barely over 1 moa. I really like the wsm's. The 300 wsm has way more choices of factory ammo then the 270 wsm, go check on MidwayUSA. And for the non reloader thats a plus. But imo, in a 30 cal you need to shoot a 208 plus gr bullet to get a decent bc. And the 300 wsm wont be real flat shooting with that heavy bullet. But a 165 gr would be a killer hunting option in the 300 wsm imo. Like its little bro the 270 wsm with a 130. Its all give and take. I go out to WY prepared to make a long shot but have never shot over 100 yards so far. Idk if its lucky or unlucky.
My 270 wsm with a 130 gr BT or Berger is almost too explosive on deer under 150 yards, am going to a Barnes 129 LRX to control this, hopefully. And my handloads are meek at only 3042 fps. Dont sell the 270 wsm short, mines killed a cow elk, 3 mullies and a handful of white tails. All 1 shot. Except a buddy didnt make a good first shot once on a mulley buck but not the rounds fault. I think we all get too caught up in efficiency when it comes to high bc's for distances usually past the average hunting shot. Because this LR efficiency comes at a cost, the flatness of the round at <500 where most shoots are made. But of the two you listed I'd go 300wsm. Hands down a better choice with more ammo choices.