use the system timer which only gets updated once every 10-16ms, depending on your hardware. For measuring time consumption of a process this time resolution is usually not sufficient.
More importantly, both methods do are not specific to a process - they just measure the global time, not the time a process uses on the CPU!
You should take a look at QueryPerformanceCounter
]. If you follow that link, you'll find a comment with a listing for a high resolution timer (class HRTimer) that refers only to the active time of the current thread.