which of these is more efficient for AC coupling
at the end I suppose they are both high pass filters, and the (20 uF capacitor ) is supposed to be connected to a load to form the filter, in this case my load is supposed to be the impedence of the the analog pin ( I guess) to which I am connecting my signal


Yes, that's right.


  but still I couldn't figure out which one will perform better in my case ?


Without using the extra external resistor you probably improve the efficiency (because the resistor converts some power to heat). 

On the other hand if you just use the Arduino input pin as the load, you probably don't get as good of control of the cut-off frequency.

If you just need to eliminate the dc offset and don't really care exactly what the cut-off frequency is, you can probably eliminate the external resistor.

One more thing you should consider is that the input range of the Arduino pin is probably something like 0 to 3.3 V or 0 to 5 V. If you don't re-bias the dc level, you will probably get a signal in a range like -1 to +1 V or -2 to +2 V, which will not interact well with the Arduino. You should probably add something like a resistor divider to re-bias the signal. And then that divider (or it's Thevenin equivalent) can also provide the "load" resistor to set the high-pass cut-off frequency.