I have considerable experience with the .35 Whelen and a few years experience with the 9.3x62. I agree that for most hunting situations they are interchangeable without a noticeable difference. In my rifles, the standard Remington factory load 250 gr. velocity is measured on my chronograph at 2400 fps. And the standard Norma 285 gr. 9.3x62 is also at 2400 fps. My Whelen's barrel length is slightly longer, about 3 cm or 1.5". So a slight power advantage to the 9.3.
I like the .35 Whelen very much for hunting our biggest Canadian game. It is very effective on elk, moose, caribou and bears.
But for international travel there is no comparison. It's the 9.3 for me, every time. Much better ammunition selection and availability, usually superior bullets, and the history and reputation favour the 9.3 too.
When i hunted Namibia last year, my PH was very satisfied with my choice of cartridges. He also shot a 9.3 and appreciated that we could use the same ammunition if necessary. And there are 9.3mm factory loads that are suitable for thick skinned dangerous game. Not so for the .35 Whelen.
I enjoy handloading ammunition and understand that both cartridges can be loaded to serve a variety of duties, and with handloads they clearly do overlap in usefulness. But to me this discussion is kinda like the old .280 Rem. vs 7x64 Brenneke comparison. Both the 9.3x62 and 7x64 did it all first, and did it better from the beginning, and continue to do so.