Simple question: MTB or CX?
This is my first year doing this ride so I’m trying to decide how seriously I want to ride or race it. I’m not new to bike racing but I’m new to this race. I’m leaning towards my hardtail aluminum MTB w/ tubeless vs carbon CX w/ tubeless.
Thx for any opinions or advice. 
Do you own both a MTB and a CX bike? If not, run what you have! If you own both, then:
What are the majority of participants riding?
What are the age-group winners riding?
Personally, I would go w/the CX bike 
I’ve done the race a few times, 3x on a cross bike and last year on a short travel trail bike ( Santa Cruz Tallboy). There is a fair amount of pavement and flat-ish gravel between the major climbs that will reward a cross bike, but there’s more chunky gravel, especially on the downhills, that reward a mountain bike.
When I did it on the mtb, I felt fresher at the end since I wasn’t getting my body beaten up by the bumps so much on the downhills, and felt way more confident on those downhills. Also, having low mtb gearing was huge, especially at the end when you’re gassed.
All that said, I’ve never really raced it per se, just tried to survive and have fun. I believe the winners are riding xc rigs these days. If I had a xc hardtail that’s what I’d be riding this year—best mix of speed, climbing ability, and comfort.
You could also add a set of aero bars to the hard tail for a little aero benefit on the flats. I’ve seen folks do that for Leadville and even saw Jeremiah Bishop do this for the SM100 (very small ones that actually looked more like bar ends mounted in the center).