I'm a tutor at COMPOS. Obviously, this reply is too late to be useful to you but I'll post it anyway to be useful to anyone looking this up in the future.
First, the dichotomy of PAT preparation vs COMPOS entrance exam is kinda false- doing the COMPOS entrance exam is good preparation for the PAT.
COMPOS is very new, only a couple of years old. If you apply to Oxford Physics, chances are that whoever is deciding whether you get in or not doesn't even know what COMPOS is. But that's likely to change over the coming years, and mentioning COMPOS in your Personal Statement or in interviews can only be a good thing.
The main advantage of doing COMPOS is that you learn physics. It's not built as a preparation for A-Levels, PAT, interviews, University, or any other sort of milestone. So in theory, you're not missing out on anything that would help you get into Oxford. But that's only the case if you actually spend your time learning physics, which is easier said than done.
I heard that the leaders of COMPOS are considering releasing questions and numeric answers for people to follow along with the course. I'd check that out if I were you.