ive seen tethering go incredibly wrong...no escape, no defense...
id personally go with a movable electric...trained form a young age they wont mess with the electric so the posts wouldnt need to be overly secure
but ijust did 4 strand electric over about 3 1/2 acres for around $100. you could easily fence in your entire property that way, save money and use the trees as posts with nail or screw in insualtors and its easy to change about. and a cow wouldn't need more than a strnd or 2...