I have two pastures with one top strand of 1.5 tape and 2 strands of polywire under that. I have never had an escape through the electric. (I did have a mare that learned to open the gate but that has nothing to do with the electric part) I used 5 1/2 foot t-posts for both, but I wish I had done 6 foot. I would also add that you should use the t-post toppers that are designed to hold the tape, like the first picture not the second.
The new pasture that I am about to do will have 6 foot posts and all 3 strands in the wide tape. I am doing the strands of tape different for asthetic reasons. Also I have sections wooden board fence around the gates. This avoids having the horses hit the electric when being lead in and out of the gate, then associating you with the shock. (I have had this problem at other places)
I have seen 2 strands of the 1/2 inch used, but they did have escape problems. I would advise against using just one strand. This is a very cheap fencing method already and adding another couple strands would not raise the price too much for the amount of fence you are doing. I would play it safe with at least 3 strands.