Typically, pipe sizes below about 16" are adhesive bonded, above that size are butt and wrap.  Both are good joints if done properly (which can be a big IF).  I would check out Smith Fiberglass website www.smithfiberglass.com , which has some information on joining systems.  I would also check the pressure rating of the pipe you installed.  Your pressures seem high for "standard" FRP piping.

The butt and strap should be used for all sizes.   While the pressures you have stated are possible with any system, they are not advised in FRP systems due to the joints.  Many of the major users of FRP (petrochem, pulp & paper) only allow butt and strap in pressure applications.  This is from years of lessons learned.   Look at the major problems with fiberglass in the offshore industry and it is normally due to the socket joint (e.g Sabine Island).  Some of the most sucessful installations were butt and strap (Mars, Ram-Powell, etc.)

Both joints have similar strengths in pure axial loading.  However, the butt and strap joint can be as high as 4 times the strength under bending load (e.g. pressure and thermal expansion at elbows).  

It is critical to realize that both joining methods only obtain a secondary physical bond because the resins used are thermosets.  No chemical cross linking occurs between the two previously cured components (e.g. pipe & elbow).  Therefore the geometry of the joint determines the strength.

The butt and strap is better because:
1.  The shear area is greater
2.  The overlay is cured over a surface which is irregular which provides physical locking that can not occur with an adhesive joint which must be shaved to fit into the joint.  Butt and straps are not perfect, but are not typically prone to the catastrophic failure of socket joints which occurs when you have a fast brittle fracture of the adhesive.
3.  It uses long continuous fibers (the strength carrying component of composites) to carry the stress from one component to the next.
4.  The degree of cure of the joint can be evaluated prior to hydro (while the adhesive in the socket joint is not accessable, except at the exposed filet).
5.  The joint can have some (limited) visual inspection for defects.

 BUTT AND WAP V.S. ADHESIVE BONDED
MR GVANBEEK:
 THANKS FOR YOU VALUABLE INFORMATION, THE MORE IMPORTANT INTITUTIONS (NAVY, API )RECCOMENDED BUTT & WRAP.FOR THE OFFSHORE "FIREWATER SYSTEM". IN THE GULF OF MEXICO(TEXAS AND LOUISIANA, THE 95.0% OF THE COMPANIES USING BUTT & WRAP) IT IS MORE RELIABLE. THANKS
                                  JABPIPING