I think generally Southampton University has a better reputation than London met, so the level of teaching, level of service from the careers department, etc, etc, may be better at Southampton. Southampton is also cheaper, for pretty much everything, apart from commuting to London, which you may need to do to attend interviews if you haven’t already scored a job.
However, living in London is more exciting (though more expensive), and you can start to build up a network, that you can further develop when you start working. Any networks you make in S’ton, you’ll have to leave when/if you move back to London.
Personally, I’d pick London, because I like it. Others may want a year in a quieter place before a lifetime in the city. Things like this really come down to personal preference - you’d certainly have a good experience at both institutions. Think about specifics of each course, and each uni that suit you personally. For example, are there certain things on a course which you find more interesting than the other course?