I've been hunting with a .35 Whelen for over20 years. Right now I am typing this from a treestand and the rifle with me is a Ruger in.....35 Whelen! I have a view here of 350 yards and made my longest shot from this stand on a whitetail @ 260 yards, DIT. The Whelen has factory loads rated at 2900 FPS, with a 200gr bullet, my rifle is loaded with them now. Never owned or shot an 8mm mag, but own a ballistic twin in factory loads, the 325 WSM. I was using it yesterday in this same stand. The .35 has more bullet options and LOTS more factory loadings, plus more frontal area, which I like. I'd look at the Whelen 1st and also the .325. If I could only own one, it would be the Whelen, which IMHO is better out to 300 yards than the 8.
Good luck with whichever u choose, now back to my hunt!
***Added after my hunt. The 8mm mag is actually not really much flatter shooting than the Whelen, looking at modern loadings. When both are zeroed at 100 yards, the 8mag will drop 11.33" at 300 yards with a 200gr bullet and yeild 2478 FPE there. The Whelen will drop 13.51" shooting a 225gr bullet and yeild 2407 FPE at that distance. Muzzle velocities are: 3,000FPS for the "8" (200gr) and 2800 FPS for the Whelen (225gr). The BCs are as follows: .426 for the 200gr "8" and .430 for the Whelen 225gr. These are both the same type bullets, Nosler Partitions. These numbers come from one of the few common factory loadings, Nosler Custom loaded ammo.