Road (or 3 bolt cleat pedals) usually have a larger surface area to spread the load, providing a stronger connection and power transference. Most road riders disengage from the pedals rarely and usually require a firm attachment to the pedal for power transfer and efficient pedalling.
What is Float? Float is the degree of movement given by the cleat, or how securely it is positioned. This allows your feet to either pivot or not when pedalling. Some cleats have no float which keep feet locked into the pedal. This may be preferable for racing, but a system with float maybe a better option. This allows joints to move freely when pedalling and help prevent strains.
When thinking about your choice of pedals you might want to consider which shoes typically accompany them. You may already have a pair of shoes with cleats and consideration must be given to the compatibility, as there is little crossover between pedals, cleats and shoes. Therefore it is best not to set your heart on a particular pair of shoes or pedals you love before finding out if they are compatible.
If opting for cleat based pedals, you may be one of the lucky ones who can attach and attack, or you may need to play around with adjusting them every few rides to get the right position for your pedalling. It is worthwhile having a cleat fit but this isn't always necessary.