I have both, although the mtb (Ridgeback MX) is modified with slicks and rack for town use. (It's used for shopping and general transport). The road bike is a tourer (Dawes Horizon).
The (front only) suspension on the mtb actually makes it less comfortable than the tourer. It does however make it more suitable for going up and down kerbs and the like, and travelling along the canal towpath, without wrecking the wheels.
On the occasions I've used the mtb for more than about 10 miles I've ended up with sore wrists and backside. The tourer I can ride all day, 50+ miles, and hardly notice apart from my leg muscles. A part of that is the difference in the saddles. If I put the tourer's saddle on the mtb if I'm using the canal to go to Bridgwater I don't get such a sore b*m.
I wouldn't want to part with either of them. The straight handlebars and tense, hard geometry of the mtb is great for buzzing around locally, whereas the tourer's gentler geometry, drops, and general feel are right for a relaxed longer ride. The tourer is also more efficient, which can be used either for greater maximum and average speed (by 15 to 20%) or to increase range/ decrease effort.
I don't find the inflation pressure of the tyres makes much difference to ride comfort. A bit to resistance, and soft tyres are good in slippery conditions.