Congratulations on your offers! I'd probably say uni A will allow you to more likely get on an econ masters course but some people do econ masters without taking econ at all. It depends what uni you want to do a masters at for example Birmingham's standard entry requirements for econ masters is:
"A 2:1 bachelor’s Honours degree (or overseas equivalent) in Economics or a related discipline. You are expected to have received a good training in economics, to at least intermediate level, and to have taken at a minimum: Microeconomics, Macroeconomics, Econometrics or Statistics, and Mathematics or a Quantitative module at bachelor’s level."
But honestly, there are plenty of masters similar to straight economics so I wouldn't worry too much and Bham uni even say that
"For students interested in this programme but who do not meet the academic subject requirements above, we offer a two-year Masters degree with Integrated Pre-Masters. Students with a 2:1 bachelor’s Honours degree in an unrelated subject area with some mathematical content will be eligible for consideration"
I would go for uni you are going to enjoy the most as they are in similar rankings and areas so doesn't matter too much. It looks like you are already leaning towards Uni B but don't rush your choice there is still plenty of time to decide. Good luck!