External clock oscillator and MCU
The 8MHz crystal uses less power and is easier to route traces for since you aren't dealing with the faster rise/fall times required for higher frequency signals. The PLL in the MCU can then step up the 8MHz to whatever frequency you actually need.
You need very exacting frequencies for some applications though where getting close woth the PLL is not enough and it needs to be an integer multiple so can be cleanly divided down.