Get the biggest you can afford. Period.
Biggest mistake I made when buying my 4s was that I had ordered a 32 GB and asked a couple of people about it, changed my mind and got a 16 gig.
Honestly, the lack of space on my phone is one of the biggest driving factors forcing my upgrade to the 5. I went with 64-gig this time around and would have gone with the 128 if the offered it.
The following is true for me (maybe you too):
-- My music collection tends to grow, not shrink.
-- The camera on these things is getting better = better quality, larger file size pictures.
-- Ditto for vids.
-- Related to the above: I tend to take more pics/vids as my kid gets older (and I'm sure when we have another) and as the quality becomes better, it makes me want to use it more.
-- I'm guessing these apps will start growing in size... just a guess though.