Hi Benjamin,
We consider using the LAPIC as a timer to be technically superior because it is closer to the CPU. The HPET is a PCH device, which means all access to the timer needs go from CPU -> Home Agent -> Fabric/IO
Ring -> DMI Controller -> PCH. Using the LAPIC cuts this path to just the Home Agent. That reduces access latency, which for a timer means higher resolution.
Hope that helps,
Nate