The difference is mainly in the low pressures commonly used in difference to variations from older non-standard chambers and '03 Springfield rifles. A modern commercial action chambered in 35 Whelen is ready for modern non-magnum pressures. The 350 Rem Mag gets 2497 fps, 250 grain, 26 inch barrel, 53,200 cup. A 35 Whelen is rated at 2503 fps, 250 grain, 24 inch barrel, 48,900 cup, both from Hodgdon load data. The difference in barrel length and pressure both favor the 350 Rem Mag, which would make the gap even wider in favor of the 35 Whelen. The 35 Whelen holds 66.4 grain of water and the 350 Rem Mag holds 62.6 grains of water, using 225 grain Nosler spitzer bullets. The difference in case volume tells the story, if bullet diameter and pressure are equal.