Hey Jim....For me...I have a 2 ski hardcase and if the airlines are going to charge me for bringing an extra bag...might as well have 2 skis in it.
But when traveling with ALice out west, I may just bring the one case with both our carving skis in it rather than pay the extra cost for the other hard case we have. Then I could rent fat skis if the powder does commeth. Having said that, if the forecast is snow, snow, snow...I'm bringing both.
No question everyone says the IM78 is an excellent all mountain ski performer. My personal preference is to have the ski that best works for the condiions I'm skiing. I believe there's a huge cross over capability between your Magnum and 78....given the 71mm waist and wide shovel and tail on the magnum.
I think for RSIII, you might consider bringing the SSMag....it think you'll get double your pleasure and double your fun....it's quicker edge to edge, more lively, and much more versatile up to boot high snow.
Now..if your in the market for a wider waist ski....hmmmmmm....

.....I just so happen to have a 84mm waisted Watea that would give you better coverage for a greater variety of snow conditions out west than the IM78.
If it were me with your skis for Steamboat...I'd bring the SS Mag without hesitation.
Best,
G