Shimano spd-sl pedals (105, ultegra, dura-ace) are going to be slightly heavier than a lot of the competitors pedals. However, they do have a wider, more stable feeling platform and seem to be a bit stiffer than a lot of other pedals. In my opinion, the wider platform and increased stiffness is worth the minimal weight penalty. Shimano pedals are also extremely durable (I've been on the same pair of ultegras since 2005) and have a replaceable plate on the pedal to keep them feeling great for years.
As far as float goes, Look caims that a majority of people are better off using 0 degree float cleats when fit properly. As previously stated, having cleats with float depends on physiology, previous knee/leg injuries, etc. The best bet here is to find a shop with a decent amount of experience in fitting cleats and have them take a look at your pedal stroke. Look makes a device that some shops have which digitally measures the best degree of float based on pedal stroke but it may be hard to find a shop that has it since it is a very expensive gadget for small LBS to invest in.