Speedplays are well known as a high quality pedal that is quite unique when compared to most every other road pedal, including the Shimano. Unlike the conventional road pedal, Speedplays will allow you to clip into either side of the pedal, which is a luxury previously reserved by clunky and uncouth mountain bike pedals.

Speedplay is able get away with this innovative design by drastically changing the cleat as well, from something like this:


to THIS!

^^Whoa.

Although you are too new to the sport to realized that you are too fat for this sport, you should note that the difference in weight between these those two pedals is somewhat deceptive. The Shimano pedals are indeed heavier than Speedplays, but the cleats they require are much lighter than Speedplay cleats.

If you are going to be a true roadie, then you are on the right track by posting these two options. Mountain bike pedals and shoes, as everyone here knows, have no place on a road bike, no matter their practicality or ease of use. That said, Speedplay would be the better choice if you're into things like practicality and ease of use. While I have never owned a pair of Speedplays, I hear that it's nice to be able to clip in to either side of the pedal, and the cleats have better traction as you tap-dance across the coffee shop pre- or post-ride. Personally, I enjoy using my non-Speedplay pedals to demonstrate my pro-level clipping skills at my local hammerfest (Hammerfest: a sort of group ride where pain takes priority), and similarly, I view walking with non-Speedplay cleats as an opportunity to practice balance and mental focus.

There are drawbacks however. No one really "owns" a pair of Speedplays. Speedplay "owners" have simply purchased the license to use the pedals and cleats, but under no circumstances are you allowed to consider yourself the owner of said pedals. View this thread for more information.