Which circuit is more Stable?
The optocoupler isolates the two sides of the circuit from each other, so the MCU is protected from whatever happens with the relay.

The two circuits differ only in the extra transistor (Q3).
Such a transistor is used when you want to use a small curent to switch a larger current; this is necessary if the optocoupler cannot pass a current that is large enough for the relay.

The 4N35 has a CTR of at least 100 %, so the LED current must be as high as the current you want to allow through pins 3/4.
Furthermore, the LED current must be smaller than 60 mA, so the 4N35's collector current must not be larger than about 50 mA. (It should be noted that the LED wears out faster with higher currents.)

If your relay allows being switched with less than 50 mA (with 12V/400Ω = 30 mA, this is the case), and if the MCU is able to drive the LED with that current (which is unlikely), you can use the second circuit. Otherwise, you have to use the transistor to amplify the current.