To save everyone from googling to try and find a more concrete difference between these two degrees it'd be cool if you included some more information.
Is the "Physics MSci" a 2 year masters program of pure research, are the 120 credits made up of mostly research based units or are they more formal classes?
Is the "BSc + MSc" an accelerated version of the "Physics MSci"? After this diploma is the next step Phd?
It sounds like the "BSc + MSc" is kind of like an honours degree.
Of course I'm probably not the best person to answer the question but I no one else has really gave it a shot, the first thing I would be doing is talking to lecturers, other post graduate students, post doctoral students, anyone around who has already been in your position.
Where I'm from a "BSc + MSci" would be a short cut into a Phd program, which means that students who decide to take this path have already planned to continue onto Phd and have pretty good grades.
The normal MSc would be chosen by a student who either couldn't get into an "Bsc + MSci" program or is planning on stopping after obtaining the MSc to go looking for jobs.
It really, really depends on what kind of jobs you are talking about as well. If you are planning on going into research within a university I would think the best path to go would be the "BSc + MSc" if you think you can obtain high enough grades to score a Phd scholarship / access into a Phd program, but at the end of the program, if you decide to stop your studies and go job hunting (which I guess is your biggest concern) I think employers would generally favour a full MSc over the post-graduate diploma. I think that is the case because you would gain a lot more experience during the full MSc program.
So yea, I think that employers, in industry type jobs, would favour the MSc, whereas if you're looking to go into research the post-graduate diploma would be the way to go (If it is possible to get into a Phd program afterwards) because you its more likely academics would hire post-doctoral students over post-masters.
Most importantly, you should talk to some of your teachers to see what they say and ask about what their past students have been doing.
Anyway, I hope it kind of helped.