Better bullet selection with the .338, and it's just gaining steam in popularity. Has an edge in BC too, but I'd be lying if I thought it was that important in a 200-300 yard hunting rifle. Reality is I have a lot of respect for a fella from another forum that recommended it after dumping a bunch of African plains game with the cartridge. 35 Whelen never really came into the mix. I was either necking up to .338 or down to 7mm (280 Rem). I'm pretty darn happy with the way I went... I'm shooting 225 grain bullets as fast as I was shooting 180's from the '06 with an almost identical trajectory (full disclosure - 24" barrel now vs. 22" before). No real change in powder volume either.
As you know, the difference between the .338-06 and 35 Whelen is pretty negligible. There's factory brass to be had, but if you're reforming .30-06 keep in mind necking up to .338 makes for some short brass. Going up to .35 is going to be even shorter. Again, not a huge deal, but very easy to see once you start.