Expedient, politic, advisable are comparable when they are used to imply a choice (as of course, action, or method) and to mean dictated by practical wisdom or by motives of prudence.
Something is expedient from which definite and usually immediate advantages accrue. Originally and still occasionally the word carries no derogatory implication.
In its sense development expedient came to imply determination by immediate conditions and to mean necessary or suitable under present circumstances. As a result expedient now commonly implies opportuneness (sometimes with a strong hint of timeserving) as well as advantageousness.
Very frequently also it connotes such an ulterior motive as self-interest.
Consequently expedient is often opposed to right, the former suggesting a choice determined by temporal ends, the latter one determined by ethical principles.
Something is politic which is the judicious course, action, or method from the practical point of view. Though often used interchangeably with expedient, politic may be applied discriminatively to choices involving tactics or the effective handling of persons, and expedient to choices involving strategy, or the gaining of objectives.
Like expedient, however, politic often implies material motives <whether it is not your interest to make them happy … Is a politic act the worse for being a generous one? —Burke >
Something is advisable which is expedient in the original, underogatory sense of that word. Advisable has now nearly lost its original derivative sense and is preferred by writers or speakers who wish to avoid any of the unpleasant implications of expedient or of politic.