Differences between current sources transistor and current mirror
In your first circuit, it's uncertain what output current you'll get for a given input voltage. First, because if you build this circuit on several chips, the PMOS behavior will not be identical from chip to chip. Also because as the temperature chagnes the PMOS device behavior will change.

In the second circuit, you know the output current will be essentially identical to the input current, regardless of variation of the MOSFET performance. The trick then is just to make a current source for the input that is insensitive to process and temperature variations.