Purely looking at pass rates I would say they are similar in difficulty.
Although paper P5 had a pass rate of 28% which is the lowest of all the optional papers and P7 is around 40%.......so maybe you could assume this one to be easier?
I am afraid from there I can be of no further help.
You may want to read up on the option papers to see how they follow on from the F papers and then choose based on your favourite F paper?
For example: paper P4 is the follow on from paper F9 so if you really struggled with F9 then maybe paper P4 would not be the one to choose.
It depends on you what subjects you like to study ??
As I chose P6 and P7.
Reason behind this selection is, P6 is an easy but little more technical because it includes percentages, rates etc to remember and I like P7, I love to take knowledge about audit in detail. So it depends on a person.
As i personally don't like F5 and P5 because i am not interested in these papers..
But P4 is a very good paper because it includes lots of knowledge about financial management.
And the last thing is that kind of job you thought for yourself after completing ACCA. If you are interested in joining FIRMS then I prefer you to study P7 and the other of your choice. But if you join Industry then P4 or P5 is good and again the other paper of your choice.
The option that I gave you is not restricted. Think by yourself by linking with the future.