I have only 2 gigs left on my 16gb 3GS, so I think I will go for the 32gb in black. Don't want it to be too obvious i'm holding an iPhone when using a white one.
I have a 32Gb 3GS right now and I could totally manage with 16Gb, but so far it looks like I'll just stick with 32Gb. I'm certainly getting black though.