Narrating it down so that I don't forget.
1) Riding an MTB is largely like riding a cruiser motorcycle. You are relatively in comfortable position and you don't need to manoeuvre to large extent unless you are into dirt biking or on a mountain trail sorta thing. Riding a race-road bike with drop down bars compels you into relatively aggressive stance and encourages you to pedal faster and harder. Does it mean you cannot ride for hours on it? No, on the contrary, you can ride longer routes and for hours subject to your stamina. That's largely because of effortless pedalling against the riding stance thing.
2) Pedalling an MTB is largely about dealing with inertia thing. Pickup is not that fast but an MTB can also give you a decent average in terms of speed subject to road conditions and that it already is in motion. Climbs are relatively difficult, slopes are fine. And tires mostly carry treads which also adds to the friction and therefore onto the inertia part. Pedalling a road bike is quite effortless and climbs are easier to do. The first two climbs of Nashik highway were climbed at 22nd speed (50T + 14T combination). Tires of a racing road bike are sleeker with no treads whatsoever. That reduces friction and makes climbs easier. Of course they will not work out on dirt trail or mountain trail sorta climb.
3) Uneven roads, stones, speedbreakers, potholes can ruin your road biking experience. You will need to learn to manoeuvre for avoiding them. Also the bumps and vibrations will be quite higher in riding a roadbike which can impact your back and wrist. MTB will ride through uneven surfaces with ease and will protect from impact courtesy suspension.
4) There are more varied options for gripping a drop down handle bar of a race-road bike as against MTB. That gives you option to change your riding stance at times and grip positions on the handlebar thereby protecting your wrist from repetitive impact on any particular point.
5) Braking quality is higher in MTB as against road bike. Road bikes cannot afford to have disc brakes as their weight creates imbalance towards left side of the frame. Rim brakes do work but they do not brake that fast, neither you would want to brake abruptly if you are riding at a very higher pace. In all, you will be happier if you get smoother emptier roads with minimal traffic on a road bike.
6) Carrying backpack is tough on a race road bike as the weight of your backpack is felt directly on your lower back. That's primarily because of your riding stance. Backpacks are easier on MTB because of cruiser position in which the load is felt on your shoulders. You will get tired faster if you are riding with a backpack on a road bike.
7) Gear shifting options are varied and different in road bike versus MTB. Shifting to smaller cogs is quite easier. Shifting to higher cogs require deeper pushing of the shifter in microshift R8.
8) Pump are different for the two bikes as the PSI required for road bike tires is way higher than MTB. My btwin MNP 300 pump cannot pressurize above 5 bar even though it has nozzle for both presta and shradaer valve. Btwin mnp 500 for road bike can fill air upto 8 bars though it is tricky pumping air in the valve that hutchinson was carrying.
Lastly, here are the pics of the components of Triban 3.