You can't just say "Both using the same power supply". You could be seriously restricting performance of one of the motors in your attempt at an "equal" comparison. Also, this would basically refer to voltage. But current plays as large if not larger role in torque. And it's very dependent on the drive. You can get 2 amp Nema 34's, and 7 amp Nema 34's, both with the same holding torque. The 7 amp motor will have far more torque at higher rpm, provided you were driving it with the optimal components. But this would likely require more expensive drives.
It's possible to get a 2000 oz-in Nema 34 that would have less torque than your 270 oz at 100 rpm, and it's also possible to get a 400 oz Nema 34 that would have much more torque at 1000 rpm.
It's all about properly matching the motor, drive, power supply, and drive mechanism. When comparing stepper motors, it's rarely apples to apples.