16-bit OS are barely seen now. If I'm not wrong, Vista no longer
supports anything 16-bit.
Vista only has 32-bit and 64-bit.
You can run 32-bit programs on 64-bit OS, but it's not true vice
versa. 16-bit support is non-existant on 64-bit Vista.
There's no hard and fast rule on choosing either. If you want to
be uber sure that your programs will run, get a 32-bit OS.
Do note that not all 32-bit programs will run under 64-bit OS.
Although by right it should work, reality is not what you
think.
Some things for you to read:
http://www.softwaretipsandtricks.com/windowsxp/articles/581/1/The-difference-between-64-and-32-bit-processors
http://blogs.zdnet.com/Ou/?p=440