In Spitzer's case, as a presumably monogamous (exclusive sexual relations with wife) married man it is clearly a better choice to pay for sex than to have an affair. As a professional relationship it goes a little beyond a trip to the hair stylist and would normally pose little if any danger to the long-term stability of his contractual relationship with his wife. Although clearly violating his presumed obligation of monogamy there seems to be much less emotional baggage in a pay for sex exchange in contrast to an affair of the heart.

If Spitzer's relationship with his wife were non-exclusive for sexual relationships but exclusive for emotional connection then a pay for sex arrangement would still be a better arrangement for the stability of his marriage than an affair.

If Spitzer's relationship with his wife were non-exclusive for sexual relationships as well as for emotional connection then either a pay for sex arrangement or an affair would be permissible. While both have consequences for a public figure an affair is more likely to come to light while a pay for sex arrangement is more immediately consequential, and both could be used for blackmail purposes or other political gain in the wrong hands.

If money in exchange for sex were not criminal then it's much less likely that it would ever become public knowledge nor would it hold as much sway as blackmail material, although in our society political figures would still have to guard against that eventuality.