PhD in Manchester vs MSc in Oxford

[Hi-]

Doing a PhD was my original plan, I applied to several universities in the UK and I was invited for an interview for a PhD at the University of Manchester (where I am doing my undergrad). However, I was placed on the reserve list and I am now waiting to know whether I will be offered a place or not.

As a plan B, I applied for a Masters at Oxford for which I received an offer. Initially, I was confident with my choice of going straight into a PhD, however, now that I have the opportunity to go to Oxford, I am very unsure on what to do. The PhD is interesting and I like the supervisor but I am also not particularly excited about the field. I feel like doing the Masters would help me have a better idea of the research area in which I want my PhD to focus on and it would give me the chance to study in a different university. However, the PhD is funded whereas for the MSc I would have to take a loan. I see the MSc as an investment for my future and I think that studying at Oxford would give me a good chance of doing a higher level PhD the year after, even though there is no guarantee. In the case that I get offered the PhD i really don't know what I would do and I feel like I made a commitment to the supervisor which would be very disappointed if I rejected the offer to do a Masters instead. I really don't know what to do.