WPH,
There isn't a nickel's worth of difference between the two in real world applications. However, if you're a handloader, the 350RM has some limitations when it comes to bullets that can be used. If your 350 is built on a medium action, like the factory Remingtons, you won't be able to use bullets like the 225 grain Nosler BT's or the Barnes 225 TSX's becaust the ogive of the bullet will have to be seated below the case mouth in order for it to work in the medium action. The Whelen is typically built on a long action, and you would have no such problems with the long, high BC bullets. Remington puts 1 in 16" twists in all their .358 rifles. The longer, heavier bullets like a 1 in 14" twist better, but there are some folks that have good luck with the slower twist, YMMV...
F1