What little Tango sheet music I've seen was written in 4/4. Most dance teachers will count in the dance for beginners classes - 1,2,3,4 (or maybe 5,6,7,8) and most people will count four beats.
However! The ISTD in its infinite wisdom defines Tango as having a time signature of 2/4. They then go on to say that a slow is one beat and a quick is half a beat, and count the bar as 1,&,2,&. In the teachers exam you have to demonstrate figures whilst counting in beats and bars, which means the first count of each bar is used to count the bars themselves. This isn't to bad for 4/4 timing, you just count 1 2 3 4 2 2 3 4 3 2 3 4 etc but for Tango they want 1 & 2 & 2 & 2 & 3 & 2 & and it gets confusing which "2" is which.