Good questions, I'm of the opinion that in the Rockie's the temperatures are mild enough that even snowmobile packed snow is often not hard enough to support bike riding. Especially when it is deep snow. If it is just a few inches on a hard surface underneath, sure, no problem. But if the snow is pretty deep there has to be pretty heavy snowmobile traffic to pack it down hard enough. And because the temperatures are mild enough and the snow is pretty dry in the rockies the snow does not usually get real hard on it's own.

Of course, the bigger the tire footprint is the better the bike will get some flotation and the wide 4 inch tires are going to be the best choice. They have significantly more contact area and can be run at much lower pressures than a 29'er tire. A 29'er tire has only a slight contact area advantage over a standard 26 inch tire of the same width. In fact, a 4 inch wide 26 inch tire has such a high profile that it is probably almost as big as a 29'er mountain bike tire on it's outer diameter.

Then there is always the option of the cross country skis if the snow is not hard enough to support the bike. I think the 4 inch tire and wide rims are the better choice for a snow bike.