There is no clear answer as to which is better. One is coaxial and the other is a right angle valve. The coaxial can open faster because the piston has a shorter travel distance and less air to dump. A typical right angle piston might have to travel 4 or 5 times farther than a coaxial piston to allow the same airflow.
In a coaxial, however, the air must make a 180 degree turn, as opposed to the 90 degree turn in the other design. So supah style valves probably have greater airflow.
Any piston valve is difficult to make. If I were going to use one I'd probably buy a Supah. At least I'd be assured the thing would work.
The valves I use are like the coaxial piston in your first link, but instead of a piston they have a thin diaphragm. It's easy to make (well.. with a little practice it's easy... the first one can be a pain), cheap, and performance is excellent (Cv = 21 for 1.5" , Cv = 43 for 2"). Here's a link:
http://mywebpages.comcast.net/glenshind ... ternal.gif
The guy at spudzooka.net might even have a few 1.5" diaphragms left to sell.. buying one saves you the aggravation of making one.
Whatever you go with, use something better than a ball valve or blowgun to dump the air behind the piston/diaphragm.