With shorter or less intense rides it shouldn't matter which system you choose. Road pedals are designed to provide a larger area to contact the shoe which should help eliminate hot spots that can develop on long rides, or intense rides, or whatever the reason my be, road pedals should have less.
When you look at the shoe's themselves you will see road shoes are pretty much just designed to ride in, MTB shoes or casual SPD shoes are designed to be able to walk, at least a bit while trying to hide the clips.
If you are just a casual rider just ride with whatever you already, at least until it gets to the point where its no longer comfortable (or to where you can't deal with it any longer).
If your comfy, and have no problems clipping in and out, and can generate decent power while riding for a while, i see no reason to buy a whole new system unless you really want one.