Decimation vs Mean in downsampling operation
What you are calling decimation is in fact downsampling: keeping one of every $k$ samples from the original signal.

Proper decimation involves a digital low-pass filter in order to eliminate all components above $\pi/k$ (or $f_s/(2k)$ in continuous time) since these components cannot be represented at the new lower sampling frequency (and will result in aliasing).

After low-pass filtering, you downsample your signal keeping one out of every $k$ samples.

The second method you propose is actually a low-pass filter followed by downsampling. But in this case, the low-pass filter you use is a moving-average filter, which is very simple, but it is not a very good low-pass filter. Gain in the pass band is very variable, and attenuation in the stop band is poor.