Registered Professional Liability Underwriter (RPLU) is an excellent educational program for those who underwrite or sell professional or management liability insurance. The RPLU program is more rigorous in that it requires learning the material covered in 13 courses and passing 12 final exams. You must also travel to their testing centers to complete the exams in a secure environment whereas the MLIS open book exams (in most states) may be completed on your own computer. While RPLU focuses on the same subject matter as the MLIS program, it may be more targeted to the knowledge needs of underwriters, whereas MLIS is designed primarily for retail (e.g., independent) agents, brokers, and risk managers. If you hold the RPLU designation, you likely will gain little additional knowledge from the MLIS program, though you may find it useful for a quick refresher course or convenient online insurance CE credit.
If you are trying to decide between RPLU and MLIS, the RPLU has the advantages of established credibility, a more rigorous curriculum covering more topics, and, especially if you are an underwriter, a curriculum more targeted to your needs. The MLIS program has the advantages of lower cost, a somewhat less demanding curriculum, testing convenience (on your own computer), and, if you are an agent, broker or risk manager, a curriculum designed with you in mind.