Originally Posted by Johnnys Zoo
Well, maybe. MTB pedals are lighter, generally cheaper. As for MTB shoes, some are pretty darn light and stiff. I guess it just depends on what you want to do with your bike and how much you want to spend.
But if you're serious about getting into road cycling, it's best to go with a wider-platform, road-specific pedal system.
When I first started riding my road bike, I used SPD pedals and shoes because it's what I had on my mountain bike. And yes, the SPD road pedals were light, and my mountain bike shoes stiff and walkable. But I found two major limitations to the MTB pedal setup:
1. It was tough to get a good pull-stroke with the SPDs: even at the highest release setting, I'd pull out; and
2. I'd get wicked hot spots on the soles of my feet due to the tiny contact patch between shoe and pedal.
Granted, I moved up the latter of road cycling intensity rather quickly, so I soon learned the limitations of my equipment. And my feet have always been problematic due, in large part, to my ski racing days and the re-shaping effects of race-fit boots during my teenage years.
So after I tried other SPD-compatible shoes, I eventually switched to SPD-SL - and the change was all positive. The pull-stroke in my pedaling was perfect without having to crank down the release on the pedals, and the hot spots went away. Furthermore, the SPD-SL cleats are perfectly walkable (and frankly, any skier should be able to walk in bike cleats - after all, y'all can walk around fine in ski boots, right?), and they are cheap to replace.
(That said, my SPD cleats on my MTB shoes have lasted me 10 years of hard use - steel is real, in that case.)
As far as being lighter, that's not usually the case. Speedplay and the carbon-body Look pedals are often lighter than almost any MTB pedal, save for the Eggbeaters.
Just my $0.02 - YMMV.