If you are going to use your bike mostly for commuting, I would like to suggest you go for cyclocross. 

The reasons are:

**Motorist Hazard**: There are always ignorant motorist who think they can save a lot of time by ignoring checking blind spots, appropriate speed at junction, quickly overtake without leaving sufficient space for cyclist. The less time you spend on the road, the less chance you get trapped by those motorists.

**Safety**: Going at speed higher than 20 mph (30 kmph) on a road with bicycle is a hazard itself. Imagine having a front tyre blew off while travelling at speed higher than 20 mph, you will have an idea how little control you have over the bike if accident was to happen over 20mph speed. 

**Road hazard in rural area**: (e.g. Oil spilled, pot holes, dead animal carcass ...) You need Braking Power, Stability when doing Emergency braking (skidding), Maneuverability (swerving)... Most of those listed, Cyclocross can offer more than a Road bike.

Thus, I see very little reason for you to choose a Longer distance, opting for Higher Speed and taking chances with Motorist.
In commuting, the first priority is safety, then enjoyment (be it higher speed, better scenery, challenging trail) comes secondary. Unfortunately for me, I learnt it the hard way until I got my best path to work place (avoid most of the road with junctions, motorist are scary with road rage at busy time, especially when you travelling to work/leaving work place)