I talked to my local store yesterday, hoping they had 32GB units, but none have arrived yet. They indicated they will work with me to exchange my 16Gb unit. We'll see how that shakes out.
I can probably make do with a 16GB S4, but prefer a 32GB to have more headroom... just in case. I expect to have this phone for the next couple of years...
Edit: out of curiosity... how much room do the 16GB GN2 phones have after the OS and bloatware are loaded? I had my GN2 for less than 10 days and don't recall.