Despite the road shoes being smaller and nicer looking,
I decided to get mountain biking pedals because they are easier than road.
We decided on Shimano shoes and Shimano SPD pedals as they're pretty mainstream, standard and popular.
Even within the Shimano SPD system, there are various choices.
I actually received lots of conflicting recommendations, eg:
"These are good as the platform is wider and you can pedal without clipping in"
"These are not so good because the plastic makes it harder to get the cleat in the pedal"
"These are good because you can use 1 side for normal shoes and the other to clip in"
"These are not so good because you have the added complication of trying to work out which side of the pedal you're trying to clip in to"
In the end I went with the double sided pedal with no extra bits.
Even though the others were deemed easier starting points.
I figured it was a bit like my decision to buy the road bike, if you know where you're headed, you could just decide to commit and make it happen.