I have both - as well as a .356 Winchester, .35 Remington and a .358 Norma. I guess I must have a thing for the .35s :twisted: .
I carried the .358 for many years for both deer and moose, and I guess I must have shot about a dozen deer with it. The performance was always good, even when I had to settle for inferior bullets.
It's primarily a reloaders cartridge - which I do -, so I can't give you any reports on the performance of factory ammo in the .358. However, I wouldn't expect that there would be a problem.
I never managed to shoot a moose with it, but I wouldn't expect there to be a problem with that either. A few years ago, I picked up my Whelen with the intention of extending the range a little, though my main thought was to be able to shoot moose with heavier bullets at similar velocities and ranges.
I don't think you can go wrong with either cartridge, so I'd advise you to pick whichever one is chambered in a rifle you like.
One piece of advice (that's clear from my signature line) is that if you're hunting in the thick stuff in Ontario's central range, I'd advise you to get a rifle in a Lever, pump or autoloading action. The liklihood of losing shots to unseen intervening brush is just too high in this part of the world to trust anything slower. Shooting opportunities in this area are usually very short in duration, so if you lose your first shot (or wound the animal) you may not get a second if you use a bolt or single shot.
Whelens are available in Remington 7600s and 7400s. 358s are available in Winchester 88s, Savage 99s and BLRs. The BLR is the only one currently manufactured (other than some special run 7600s available only in the US).