Hey, Dr. Med.
I took two NBME's for Step 2 (the first two tests), and scored the *exact* same on both of them. I haven't received my real score yet, but I felt like it was exactly like the NBME's.
I really doubt there is a large variation between the various tests, so I'd just take any of them. Eenie meenie minie moe!
On another note, in my opinion you should have taken an NBME at least a week before your exam. In any case, take the practice exam, and if you don't pass it with a comfortable margin, then I'd advise you to push back your date...even if it costs you an arm and leg (since you have less than five business days). [Actually I don't think it will cost you an arm and leg...I think something like $160 or so, which is not bad...but I'm not sure, so look into it.]
Only take the exam after you've done decent on the NBME. It's the most accurate assessment of how you'll do on the real thing, so don't take the real thing if you are failing the NBME.
Good luck.