I've got both.

In a nutshell, the spd's/mtb shoes let you more easily transition from walking to biking, i.e. going to excuse yourself on the side of a bike path, going in and out of coffee shops or even when you just get off the bike to fix a flat.

The road pedals/shoes allow easier clipping in and give better foot support when you get out of saddle to sprint, climb or esp both when you want to sprint up and over a short hill where you don't want to sit and spin. They're awkward the second you get off the bike ,though, even just to change a flat and the 3-point pedals, which are also called Look style, are what you walk on unless you put cleat covers on ever single time you get off the bike which you won't do because you'll get sick of doing so.

I think it's mostly how frequently you're getting on and off the bike that matters.