I was going to start a topic about Practical Musicianship, actually, because a few days ago I found some past papers at home (grades 6-8, which I had bought because of the Advanced Certificate tests included in the same booklet) and thought they looked insanely hard compared to theory. I think Practical Musicianship is only for people who are very talented at Aural and find it easy - how have you found that so far? I am not talented at Aural and therefore found the theory much easier, but I think Practical Musicianship would be great for anyone who can do it, perhaps singers would have an advantage there? Doing grade five theory also gave me an advantage for later work such as GCSE music, although had I done Practical Musicianship it would have given me an advantage for Aural tests at higher grades of practical.
I think looking at the materials for both (such as past papers and the syllabus) and discussing it with your teacher would really help you decide. Also, don't think of Practical Musicianship and Theory as mutually exclusive - there's nothing to stop you doing both, should you so choose.