I also think that 18 inches is a more versatile length, but I do have a (fake) strand of 16 inchers to wear as more a choker. For everyday wear/investment to wear in the future I'd go 18 as well. Perhaps you could go try some different lengths on in a department store/jewelry store? Pearls lie a bit differently than a simple chain. Of course ultimately it is about what you prefer, but you should get an idea of the two lengths on you so you can properly decide.
I love my 36 inch strand. (I finally measured!) I find it's wonderful because I can either double it and have an 18 double pearl strand, a 16 inch choker with a 20 inch necklace (totally different look) or a long strand. I wear them long the most, followed by the other two styles. It gives me a lot of choices-something else to consider!