I went to Marshall for undergrad and had an amazing time. Its a great program and one that is very reputable on the West Coast. The same is true for the MBA program. But again, this is true for the West Coast and, especially, within California. Once you venture East, USC and Marshall are less known (I still don't understand why although we are ranked quite well nationally...I think USC doesn't market itself outside of California/Asia as well as it could...plus, USC alum tend to stay in California or NYC within the US...after all, why would anyone want to leave California??
In terms of whether or not an MBA would help...it depends, what kind of law do you want to go into? If its securities law or something related to finance, then I think an MBA is useful and something that can make you stand out. But the next big question to consider is WHERE do you want to work? Again, if its California, Marshall is quite prestigious there and I would say go for it and don't look back. If its on the East Coast, however, you might be better off considering UCLA just because the brand recognition is better.
Top 20 law school isn't the same as a top 20 business school. A top 20 law school, in my opinion, carries more weight than a top 20 business school. But again, thats just my opinion and based on my personal experiences with friends.
Best of luck.