WLL vs MB is not really an apples for apples match.
WLL is THE Davis logging solution for the future and I expect all future data handling features to focus around WLL. But WLL intrinsically involves uploads to weatherlink.com and most of the future innovation will happen on the cloud platform. (Though an increasing number of programs are able to talk to WLL locally, so local options will continue to expand but developed & supported by third parties and not by Davis.) MB OTOH is more of a device that provides maximum flexibility for data handling at a local level (plus any uploads that you may have MB configured to make).
So I guess it depends whether your interest is more in playing with local data handling or, alternatively, having a large chunk of the data handling taken care of for you (but potentially with a $50 annual subscription) by a professionally-managed cloud solution that will continue to have more and more features added as time passes.