I prefer front firing for general sub use. The upper harmonics are filtered out somewhat in a downfiring design by the floor so you can get a hole in the response if used with a mini monitor or planar with limited LF extension.
The floor firing works great for 50hz and down assuming the mains have a respectable bottom end. Downfiring subs tend to dissapear a little better too. REL has had a lot of success with downfiring designs.