If you want a unique experience, head up to Barrancas de Belgrano and visit the Milonga de la Glorieta. It's outdoors and basically just people in the neighborhood getting together for tango in a public space. Look for the big gazebo at the top of the hill. Starts around 7PM and goes until 10 or 11ish.
It's not close to San Telmo though. But it's out of the main tourist drag and would be a more 'authentic' experience perhaps.
Tonight you can go to Salon Canning at Canning and Cabrera. You can also go to La Viruta. Between 12 up to 3.30 its tango, rock and roll, salsa. After that only tango. It is more than a disco than a Milonga. Regards