This largely depends on the duration of your trip and the kind of road you will be cycling. I’ve toured on mountain, road, and touring bikes. You might think the best choice is the later, right? …not always.
I started my European tour crossing Spain from the border of France to Santiago de Compostela, yes that was the Camino de Santiago (aka Saint James way), with a touring bike and I wished I had a mountain one. Although I had the perfect bike for the main part of the trip which was paved, going through the unpaved trails was fun but probably unnecessary. Since I had no suspension the spokes and hubs carried the load and sent me the costly bill months of cycling later.
Here are some of the main option of bikes you will face.
Mountain bikes are great if more than half of your ride consists of trails or you don’t really know what is going to be about. Otherwise If your tour is a week or less long this might be the right bike for you.
I met a guy in Spain who started cycling in Modena, Italy, and finished his ride in Portugal within a very short time window, he had no racks and only carried his camel bag on his back… Oh yes, I forgot to say that his wife was driving a van more or less parallel to him carrying all kind of equipment, tools, beverages, etc. If you can afford this style, go and do it!
The good thing of these bikes is that you can choose any road you want to take. The downside is that the bike is rather heavy, the tires will slow you down on paved roads, and your hands will only be in one or two positions on the handlebars.
Road bikes are super light, easy to accelerate, brake, and maneuver on paved roads. You can go as far as you want provided that someone is carrying your stuff on a SAG wagon. Whether is meant for short races or larger distances, the design of this bike is not meant to comfortably carry you for touring.
I’ve done many 2 to 5 day traveling on road bikes, sometimes with a SAG van and others with my backpack. I definitely enjoy those hangouts or solo travels, but they are rather long weekends instead of bike touring. SAG vans are really nice to give you anything you need almost any time, but you wont have much room to improvise and jump into a river that came across your path. Backpacks are ok for short distances, just be ready for a non-stop sweating back and wet contents.
Being aerodynamic, fast and furious is great, but on long distances might be paid by butt, back, and / or shoulder pain. The thin tires are more prone to pinch, at least two inner tubes are needed for these trips. Racks wont fit on this vehicle, and definitely you will let the fun trail roads for a different day.
Touring bikes are the ideal compromise for most of the cycling trips as they are light but sturdy, and specially designed to carry load. The wider-than-road-bike tires will let you ride on trail roads every now and then. They are comfortable to ride for long hours as your position will be more upright compared to road bikes.
Maybe I’m biassed because I’ve travelled the most in Salsa (my touring bike), but the feeling is that one of being home no matter where you go in the world. You get used to where and how to sit on different conditions -downhills, uphills, cruising-. You know which pannier contains food, camping gear, or tools. You know how it will react on different roads. With one hand on the handlebar you get to see where you are in the map while cruising -fixing your smartphone on the handlebar is the smartest thing you can do-. You get a dissent chance for space if you accommodate your stuff wisely.
Occasionally, when the wind blows in your face, going downhill or uphill, you have the choice to be more aerodynamic and place your hands in the lower side of the handlebars offering less air resistance. Their stability is amazing once you add your panniers.
Let’s put it this way, if you had a Jeep Cherokee, a Mini-Cooper, and a BMW station-wagon. Where would you take each of them?