My 9th ATB this year. 6 on a slick shod MTB & the other previous 2 on a roadie.
All are fun, the MTB is slightly slower on average speed but I would stand up & say it's more comfortable. Roadies do have their place, but it's pointless to just have one for this one ride & let it go wanting the rest of the year. MTBs are far more flexible & can be modded slightly for everything.