I know that it is the 4200 and the 5300, but that seems to be the only difference. The price is the same. I think they are both compatable with my laptop, so why get one over the other and why no price difference?

4GB kit (2GBx2)  

CT533345    DDR2 PC2-4200 • CL=4 • Unbuffered • NON-ECC • DDR2-533 • 1.8V • 256Meg x 64

 4GB kit (2GBx2)  

CT533355    DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64