Honestly I'll throw some support behind a 16 gig one.
My iPhone is 16 gig, as is my iPad. What ends up happening in my case is I end up using my iPhone for some very specific things like listening to music at the gym or GPS. It's small and convenient.
My iPad isn't practical for walking around listening to music, so it doesn't get any music. I use Netflix a lot, which is a streaming service, and along with iCloud, a lot of digital movies are sitting there, and I end up redownloading when I need to.
In my case, if I end up watching a movie, it's usually a blu-ray on a huge TV. I know that might not help for VOD users, but I do use my AppleTV in my bedroom (sorry, but Apple quality encodes just look like crap on my home theater).
Now if you're using your iPad as a possible laptop replacement, then go 64 gigs.
Also depends on your financial situation. If going to 32 gigs is good for you, then by all means, but if you go to 64 and stuck on a noodle diet, then might need to think about things.
Oh BTW: I'm a developer, and my last app which was graphics heavy, did increase in size for retina, but it didn't double. We were just smart how we sliced the graphics. In fact, we did a little trick that I can't mention that only ballooned the size slightly. In fact we started testing it on the original version of the app.
Of course not not every developer will do what we did, so sizes could vary. Sadly most of us don't even take advantage of the dual cores.