I was in the same situation for a long time, read tons of threads and still could not decide between 13s and 16s.
Sometimes I thought I would go for JH13s because I like transparent and neutral sound most of the time, but just can not stop worrying it lack the very last bit of bass for me. And sometimes I was leaning towards JH16 because it got a lot praise as well, in which case I could get plenty of bass without losing any details on the mid and top, however still afraid the bass will be too much for me. I love tight and punchy bass but not excessive. The more I read, the deeper I got into the mess.
It turned out to be the most difficult decision for me in recent 2 months because I could only afford one of them, spend other money on a better source is much much worthy than have both 13s and 16s IMO.
All this mess ends today! No single bit of hesitation or struggle anymore! I just ordered the JH13s and sent my ear impressions to JHA.
Here's why:
I luckily found an audiologist in my city that do the ear impressions for me and at the same time willing to provide me both 13s and 16s for audition. Sadly I was not able to bring my HP-P1 with me but just iPhone4, but that was enough for me to figure out what I want.
I played Valerie's Fever, Rebecca's Spanish Harlem, Mating Dance from Dancing Through the Walls, and some random tracks from Sara and my classical albums, these are a mix of 320k MP3's and AIFF's (sorry for my phone's capacity)...
Almost on all these tracks, the 13s sounded way better than 16s. I was shocked by the difference (I am not a typically analytical head but the difference is very noticeable to me).
The sound on 13s is much faster, clearer, more transparent, neutral with a slightly boost in the bass. The treble is very smooth, I can not hear any harsh or sibilant sound, the mids are just vivid with great imaging, so real... And most importantly, the bass is ROBUST, not lacking a bit for a great musical presentation, it's there, very full and with great texture and tightened, I am really impressed how well the 13s deal with the bass.
The 16s on the other hand, far below my expectation. The highs on 16s are par with 13th, but the mids are clearly contaminated a little by the exaggerated bass, not too much to stop me listening the music but enough to be noticeable by anyone with normal hearing. And the bass is always there no matter you need it or not, this may be due to the iPhone4's feed, but the quantity is too much to make the music unreal for me, hard for me to enjoy them. Compared to 13s, the bass on 16s seems like un-controlled and bleeding around. And the 16s also turned out to be slower and less transparent.
They are just have very different sound signature on the iPhone. However, I believe if fed by a better source and appropriate amplification, the 16s will sound way better than from my iPhone, the bass may be controlled well and mids and highs are possibly very similar or same as 13s, but the quantity of bass is just over exaggerated for me.
I have to say all my impressions are based on 1 hour demo of both IEMs plugged into my iPhone4 and limited tracks played due to it's an iPhone. However, it is enough for me to get the idea of their overall sound signatures and believe I will still prefer 13s when amped. make my choice.
So the base line from me is, if you can enjoy Dr. Dre's Beats, go for the 16s and you will love them a lot because on top of the bass you will also get top notch mids and highs quality assume you are not just using them with your iDevice. If you hate Beats and sometimes will not carry the portable dac+amp stack, 13s no brainer.
I am very happy I made the trip to my audiologist today, now it's a waiting game on my JH13's