8125 is a bit faster than 452X, but it has just a slight bit of stretch to it. There is not a lot of stretch, and it usually settles in after about !00 shots. 8125 has been accused of stretching a bit when shooting in very hot weather. 452X has almost zero stretch and is not as affected by heat, but it is a few fps slower than 8125. Most people perfer 452X over 8125 but it's a matter of choice. Many shooters like a string made of 8125, for speed, and cables made of 452X to insure that the cams stay timed.
My choice over the two would be 452X. It is a very strong and stable material and is not enough slower than 8125 to make any significant difference.
Automan