1) No University will be impressed by anyone who left a degree halfway through.
2) You have guaranteed funding via SF for your MSci year. For a postgrad MSc you will have to fight for funding (see point 1).
3) Having an MSc from Oxford, or anywhere else, is not a guarantee of an immediate job, or lifelong happiness.
Stick with the MSci - get some amazing vacation work experience, work hard for a First, and see where life takes you.