Well put Crazy-legs.
My palmeres: probably a hundred 200km audaxes since 2005. SR (200, 300, 400 and 600k) every year since 2006. PBP. LEL. Blah blah blah.
I would happily ride 200k on my hardtail MTB with slicks on, but that's because I'm well adapted to riding those distances – both physically and mentally. Don't underestimate the mental challenge of being on the bike for 8 hours, potentially on your own.
The point is for the OP it'll be a trade-off: potentially more comfort on the MTB but slower so you'll spend longer on the bike. This probably means at the end of the day you'll finish in similar comfort on the road bike. If it was me I'd definately take the road bike as long as it was a nice day – audaxers are a funny bunch if you don't fall in line with a few of their rules, mudguards on wet days being one of them.
If you've never done 100km on a bike then I wouldn't leap straight into a 200. 100s can be much more social than 200s, and probably will have a larger number of entrants hence people to ride with.