There's really no difference in performance. 16 is just mostly for cosmetic appeal. I have both. The way a whip "feels" is unique to each whip. To your question, I'd suggest a 12p. Once you put some time into it, and perhaps get a chance to throw some other peoples, you'll get an idea of what you like. Personally, I have a couple I'll let anyone throw, the rest I'd only let someone with some experience throw. If you do get a kangaroo, break it in slowly. Just throw it medium, occasionally hard and/or crack. Don't put any oil or any substance on it. Just throw it a million times. Axel's (a maker) don't need a lot of break in, most other quality ones do. I highly suggest the FetLife group - Single Tail Whips Addicted To Crack. Read all the stickies at the top. TONS of great information there.
Well, I prefer to learn by experiment - but I might have one question before ordering: 16 plait should be more flexible, so I guess more "effective" than 12, right? Is that worth it?