Nylon shank road shoes with foot correction provided by shimano or similar custom footbeds with tuneable arch support, and varus/ valgus wedging as part of a comprehensive bike fit will provide real and measurable advantages to rider comfort, production of power and minimisation of injury, compared to fancy carbon shoes with no correction or fitting.
For me? Spd for the mountain bike, spd-sl for the road bike with blue (2 degree float) cleats; massive difference in riding performance, especially if you ride hard and like sprints and hills
On the MTB the cheap SPD 520 work fine, the 6800 ultegra on the road bike are bombproof and have great feel under foot, I found the 550 and 105 did not last the distance with bearings going sloppy quite quickly