What's the best twist ..?

1:12
1:14

It's got much less to do with caliber than with bullet bearing length and speed. A 35 Rem shooting 200g bullets will do fine with a faster twist, while a 358 STA shooting 280g bullets will prefer the slower twist.
Most Whelens are 14" and people love them, but many can't get the really heavy pills to shoot straight. I've heard of a few with 12" twist that don't have that issue.
Bear in mind that bearing length isn't necessarily commensurate with weight. You can have a heavy spitzer with the same bearing length as a light RN, though the RN will be going faster generally.
Hope that thoroughly confused you!

Well, my choice si the 1 in 12" twist. A 1 in 14" will work reasonably well, but when it comes to the heavier bullets, my choice is 1 in 12.
I'd have to hunt for it, but there is an article by C. E. "Ed" Harris on the .35 Whelen in, IIRC, an early 1970s American Rifleman. In it, he brings out the point that the .35 Whelen was supposed to be a "Poor man's .375 H&H". Magnum length actions for the .375 H&H were $200 or more in the 1920s and James Howe designed the Whelen as a round that would work in common 30-06 length actions. He and Col. Whelen used the 1 in 12" twist for use of 250 to 300 gr. bullets. The 275 gr. bullet was popular for the big and nasties.
I have thre rifles in .35 Whelen. The Ruger 77 and Remington 700 have 1 in 16" twists and frankly, bullets heavier than 250 gr. won't shoot worth a damn for accuracy in either rifle, and I feel accuracy could be better with 250 gr. bullets if the twist was faster. My custom Mauser has a 1 in 14" twist and while it does OK, I think a 1 in 12" would have been better. So, rifle number four is now in the planning stage.
FWIW, I have a friend who rebarreled a Ruger #1 single shot to .35 Whelen and specified a 1 in 10" twist. He says accuracy is superb with that rifle.
While this is a bit off the subject, when Winchester brought out the .358 Win., they used a 1 in 12" twist. Savage and Browning when they made rifles in .358 Win. also used a 1 in 12" twist. Then, Ruger brought out a few rifles in .358, and used a 1 in 16" twist. I have a Savage 99, an early Browning BLR, two Ruger 77s, and a Kodiak Mauser all in .358 Win. The Mauser has a 1 in 16" twist. The two lever action rifles will outshoot the bolt guns by a substantial amount. Both lever guns are under an inch at 100 yards, while the bolt guns struggle to stay under 1.5".
The two .35 Whelens with the 1 in 16" twist again are hard put to hold 1.5" with the 250 gr. factory ammo while the Mauser is just a hair larger than one inch. I don't shoot 200 gr. bullets in the Whelen or the .358.
Just my thoughts based on about 15 years playing withthe .35 calibers.
Paul B.