The 48K looks more interesting with two major climbing sections, one of which comes when you’re tired. Holmenkollen would be a beautiful starting line too!
It depends on your other goals for spring/summer. The shorter race would offer more opportunities to train for speed over endurance.
Either way, I’d add more strength training to your routine, both to prevent injury and to reduce the amount of running you need to do. Some of the best ultrarunners in the world do very little running in winter, substituting XC skiing and ski mountaineering, so you can go ahead and enjoy the snow!
When I was an ultrarunner I did about 80% of my training on the bike.
Here's a longer term goal for you: https://www.wrc2020.com/
I think it's going to be a while before I do another 24hr rogaine. Hard to imagine deliberately depriving myself of a night of sleep these days!