Everybody have told me that 16 Mhz is better if you working with time precision like a clock
That's not true.
I think both of you missed the point of why 16MHz is better than 8MHz for precision timing...
It has nothing to do with the frequency per-say, but what is generating it. 16MHz implies a 16MHz clock crystal which is nice and accurate (or to a lesser extent a resonator, which is a bit less accurate). 8MHz implies the internal oscillator (as stated in the original post), which is highly inaccurate unless tuned and even then it is very unstable when temperature and voltage fluctuations are added in.
Of course if you used an 8MHz crystal, then yes, the only real difference is that one allows you to do twice as much in the same time. Though if you are using timers, it lowers the maximum frequency of the timer.