3 year degree or integrated masters?

Hi all,
I'm looking to apply for a degree in Biological Sciences at Oxford, Imperial, Bristol, Cardiff and York. At Oxford (A*AA) and Imperial (AAA) only a 3 year degree is offered, but Bristol, Cardiff and York all offer either BSc's or integrated masters.

For the BSc, the entry requirements are:
Bristol - AAB
Cardiff - AAB/ABB
York - AAB

For the Masters, the entry requirements are:Bristol - AAACardiff - AAAYork - AAAFrom the open days I've been to, the admissions tutors have all said it's better to apply for the integrated masters as you can always change your mind afterwards if you want to drop back down to the BSc. However, I'm worried that I'd be less likely to get an offer if I applied for the Masters courses as they have less places available? Also, it could be risky to not have offers below AAA, just in case things went wrong in my A2 exams. Any advice??