While jealousy is indeed a large problem for people considering a threesome the real issue after the deed is done is not knowing where to go from there. Jealousy tends to be more of a fear than a reality so I would suggest that you formulate a plan to deal with your jealousy (since you can't deal with your partner's emotional issues for them or even really plan for them) and then discuss what happens after the encounter.

Both of you should choose the person, but you should think about the other person's feelings as well as your own. Are you offering only one "night of passion" or a possibility of something more? What does this other person want or need from the encounter? This is a person you are talking about not a sex toy so my advice from YEARS of experience on both sides of the fence is to forget your fears about jealousy and the trauma it will cause and focus on the reason you are wanting this experience.

If what you want is to enhance your sex life as a couple then I would suggest a foursome or a soft swap with another committed couple. Chances are with another couple your goals will be similar, ie: you want an experience not a life change. With a singleton you are looking for something that is so mythical we call it a unicorn- a totally sexually driven individual that wants nothing but the sex.
If you investigate the swinging community you might find someone who has a firm commitment with her partner and is simply willing to have sex without anything further.

I know the popular belief is that jealousy is the only thing you need to be worried about but the reality is that not thinking about the needs and desires of the third person causes more problems than jealousy between the committed couple.