20mm or 40mm tapes have become synonymous with electric fencing for horses. The general perception maintains that horses are incapable of seeing the fences despite there being solid scientific analysis that horses will see better than humans, if you are able to see it - horses most certainly be able to see it. This work was carried out by Dr Evelyn B. Hanggi, M.S., Ph.D where many myths are dispelled.
One of the major problems with tape is the effect that wind has on it. As tape has a broad surface it reacts to wind by flapping, this flapping causes the little metal filaments to bend back and forth so breaking as shown in this study. The metal fatigue is clear in the image.
Solid wire has the best conducting characteristics of all types of conductors and will last far longer than any. - the only perceived image is that it is difficult to see so should not be used as the horse will bumble into it and get caught up.
Traditional post and rail fences are very popular but often need to be reinforced to prevent horses leaning on them and making them go out of alignment or completely knocked over. Electric fencing is the best solution normally utilising out riders and here wire is the best option. Visibility is not an issue and you want the best effect.
Wire may also be used with tape fences where it is used in conjunction with the tape. Fortunately wire is cheap so running wire with the tape is viable - particularly if the tape has come to the end of its life. Here the wire is carrying the current and the tape is simply providing the visibilty.