452x is SK75 dyneema and is blended with vectran. It is a 66% dyneema and 33% vectran blend. 20-24 strands for a compound.
X99 is SK99 dyneema and is blended with vectran. It is a 83% dyneema and 17% vectran blend. 24-28 strands for a compound.
Since X99 is using a stronger dyneema, they can use a lower percentage of vectran and still retain it's no-creep qualities. X99 is slightly smaller in diameter as well and should make a better looking string. I have not used the X99 but would think it would be just as stable as 452x and have a little softer shot. It shouldn't fuzz as much as 452x either. If you take out the vectran out of the X99, then you have the mercury material. Mercury is Sk99 dyneema and I believe it would be very stable as well. The newer dyneema is very good. It is very stable and has a long life. It has great speed and a soft shot. Vectran is a heavier material and has no elasticity. It also is absorbent and will soak up the wax. It can become heavy do to a lot of waxing. But vectran doesn't stretch and creep. It is very stable. When 452x came out, the SK75 dyneema wasn't as strong as today's SK99 dyneema and needed the stability of the vectran added. I personally don't think it is necessary to blend vectran with today's stronger dyneema. I have had no issues with strings built out of today's dyneema.