Hi. @Guesswho has already done a pretty wonderful job of laying out the facts of your intended career path as well as the various pros and cons to pursuing a master's prior to applying to law school.
Edited to add: if you're asking which degree is more valuable to your personal and professional growth, I'm afraid that's a question that only you can answer. But if you're asking in terms of what is valuable in the legal world...my two cents (as someone currently working in law, with several Ivy League-educated lawyers in the family, some of whom conducted law school interviews for years, in case any of that matters to you) is this: don't bother. For what you want to do, work experience in a field somehow related to your intended work after law school--at a law firm, for example, or in consulting, or for a nonprofit doing pro bono work, etc.--is much more likely to impress law schools these days than a master's in history ever would. To me, it's throwing time and money away when you could be gaining valuable pre-law school work experience and actually making money for yourself (law school ain't cheap!). Sorry if that sounds harsh, but I'm only saying this based on experience. My cousin deferred entry to Harvard law to work for two years at a top consulting firm, and he will have only ever finished undergrad prior to matriculating at Harvard. This is not an uncommon route--undergrad, work, law school. If this master's is valuable to you for personal or otherwise undisclosed reasons, by all means--go for it! But it is absolutely unnecessary and unlikely to make an appreciable difference in the admissions stage. Good luck.