To throw my two cents in, I have a sako m995 trg/s that I will be building off of. Mine happened to start life as a .338 win mag. For cost sake I would go with the 338RUM, but the availability of the weatherby ammo is much better. I also reload, but am slightly intimidated with the idea of necking up 300RUM to make the 338 edge ammo. The 338 run is within the range to fit accuracy international CIP length mags which are used in several chassis like the KRG W3C folder that I plan to use on my sako. I prefer the sako to the weatherby action due to the 3 large lugs on the bolt rather than having 9 small ones. Both have the 60 degree bolt throw but I see much more strength and the action was developed for large 338s I.e. .338 LM. Not to mention that the bolt knob on the TRG is straight and is basically a bolt, so definitely easy to upgrade.
Good luck, Leadfarmer