My wife is getting a new flat bar roadie, so I need to see how fast she can go.
I can't see her keeping up, with me on my roadie. But she may be too quick for me to ride the MTB.
I used my full suspension MTB on the last ride in the west and never had any trouble getting in to camp with plenty of time (and I was always one of the last to pack up in the morning).
I used the MTB because I was worried that the roads were a bit rough, but needn't have worried. Except for a couple of gravel sections that were a couple of km each. Whether there are any gravel section this time, I don't know.
I would prefer to take the MTB to make it a bit more challenging, there are a few short days in there.