I have the Realflex and saw the video on the Smoothflex so decided to get it.  It is quite a bit cheaper than the Realflex.  Construction quality is similar.  The differences lie in the sole.  It is a firmer foam that is used.  It also has less nodes and a more wider bottom.  Definitely flexibility of the sole is less.  There is also more rubber lining the foam at the very toe, slightly less in the forefoot and about the same in the heel.  I find that in the Realflex, areas without rubber wears much faster.  I would guess that the sole on the Smoothflex would be a little more durable.  You definitely lose a little performance with the shoe because of the added weight and diminished flex in the sole but this is still an excellent shoe and great for casual jogging or training.  I will say that the Realflex/Smoothflex line are my favorite running shoes that I have ever owned in my life!