I felt my scores were more dependent on my mental state when I took the practice exams. Sometimes I just felt foggy and didn't do well even if it was an "easy" form. Luckily not he real one I felt sharp and my score reflected that.
I dont know... does it matter which one you take? Your knowledge is what it is at this point and a form that makes you feel either better or worse doesn't really matter.
The UWSAs and 15-18 all felt pretty representative to me. 18 is supposedly the hardest, but the one I did the best on. I ultimately scored an average of what I got on all of them excluding the first one that was several weeks earlier than the rest (12).