We'll I'll say this about Diamond VS CP at least... we are now WD with both & have used a bunch of both all in Customs.
My impression after having Diamond do a couple sets of their "Maximum lightening" package..... it's pretty.... but they could go alot further especially in the strutting webbing & skirt ribbing. CP definitely without a doubt makes a lighter piece when doing a max lightened piston.... My impressions are Diamond lightens, but stays very "safe" with their max lightening.... CP just goes hog wild... JE very similar to CP in their lightening. CP will pull alot out of the "belt" area & their forgings are much lighter in the way they rib the wristpin to skirt on the "X" style forgings.
On a basic custom I actually did two customs in the exact same specs, one from Diamond, one from CP exact same dimensions for 2 identical circle track motors.......Flat tops, same reliefs, same ring pack, same pin size/width, same bore, both full round skirts.... Everything the same, NO lightening options... Off top of my head the Diamond was like 510 grams, the CP was like 475. I was amazed by the difference in weight...
The CP's cost more
Almost everything I do anymore has Diamonds in it since the Jan 2008 CP price increases. Unless I need that lighter weight, Diamond can get me a piece just as strong & accurate, for less $ & most importantly do it quicker. CP is nice with the Expedite stuff when you need it real real fast but $300 for one week expedite is harsh. Diamond can usually knock customs out in 10-15 days depending on how busy they are. If you don't expedite CP's... in Winter you can be waiting a good 4-5 weeks.