I agree with David, but I will clarify, in case you are not much of a bike tech person:
Get the mountainbike. You will very likely need that low gear often. You will still have plenty high of a gear for faster road sections. In fact, it might still not be low enough, if the rear is only 32, your low gear would be 24/32(0.75 ratio). For comparison, many bikepackers run 20/34 or 22/36(0.61 ratio), or even 20/36 with a 29" wheel.
Get the mtb. You will have tire clearance for muddy conditions or the space to run bigger tires for less rolling resistance, more comfort and more control. You can always run skinnier tires in a bike with lot's of room, but not vice versa.
Can you afford a bike with 9 speeds in the rear? Not only will that give you a lower gear, it will also likely be better performing as well.