One Bull
Pros: Steady lover will get to know her and your needs. In a 'safe' relationship, tested and verified, condom use can be dispensed with. You will know who she is with and that she is safe.
Cons: With one Bull serving the needs of the wife, she may become emotionally attached to the Bull, in some instances this can threaten the marriage bond on both a physical and psychological level. Danger territory.
Multiple Bulls
Pros: variety for the wife, more of a field to choose from, more taboo, more intensity of the thrill of serving a new bull.
Cons: Condom use is a must until 'safe' is tested and verified. The unknown. You don't know how she will be treated with each new Bull. A need to establish ground rules, rituals and limitations with each new Bull---it can be simple or complex given each couple's needs.
Basic Rule: If she is susceptible to falling on love with a man who is fucking her on a regular basis, to the strength and dominance of a Bull, if she is passive/submissive in any serious way---switch up the Bulls on a regular basis.