If you have a lot of business experience, an executive MBA may make more sense. Also, if you like what you're doing and want to keep doing it, then an exec MBA may be a better fit. But, if you're looking to make a change in your career or have less actual business experience (and more science/engineering experience, for example), then a part-time MBA may make more sense.
Also, some companies will sponsor their employees in one program or another. Usually, companies are more willing to sponsor their employees in exec MBA programs, since they're investing in that person's management potential. But some companies are also willing to sponsor employees in part-time programs.
Scott
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Co-Author, Your MBA Game Plan