IIRC, crossbar memory architecture doesn't enable on geforce3+ if you are in 16 bit color, so there is little advantage in choosing it. Geforce 2 GTS or GTS-V, on the other hand, are really bandwidth limited and 16 bit color can let them fly.
The most noticeable disadvantage of 16 bit color is often in particle effects such as smoke which appear dithered (splotchy dots) rather than smooth as in 32 bit color. In older games such as half life where walls are often shades of gray, there often won't be enough colors to shade them smoothly. On the other hand, in something like Max Payne with very high rez textures, this will be less noticeable because the dithering will blend into the woodwork, if you will.
On the other hand, different game engines handle this differently. Quake III powered games (elite force, alice, heavy metal fakk2) can look quite decent in 16 bit color, while on the other hand, Unreal powered games (Deus Ex, Undying, Wheel of Time) can look quite craptastic in anything less than 32 bit color.
I used 16 bit color most of the time on my Geforce 2 GTS (which could have free FSAA and still have frames to spare in contrast to using 32 bit color at the same resolution) but on my Geforce 3, I've come to love, appreciate, and require 32 bit color whenever possible.