If you go back in some gun mags you'll find some of the old "338 fed vs. 358 win shoot out!" articles. Usually there's some small differences that only the author of the article would find valid. The same goes for the 338/06 vs. the 35 whelen.
Ruger has recently carried both the 338 fed and 358 winchester in their hawkeye rifles and in their 'frontier' models. Both shoot similar weight bullets, but the 338 has a slight edge with ballistic coefficents...the 358 makes slightly bigger holes, and might do a bit better with 250 grain bullets at closer range.
I shoot 225 grain nosler partitions.
Kurt