A friend was shopping for an iPad the other day and asked the 2 question that seem to always come up (not white or black). How many GB do I need and do I want 3G service.
My first iPad did not have 3G as I figured that I had a verizon MiFi hotspot and there was Wifi all over so I didn't need it. That worked great until I found myself on a business trip with the MiFi not working properly and no Wifi anywhere to be found. I would gladly have subscribed to a month of the 3G service in order to have internet access, but I couldn't because I did not have the 3G model. The lesson, hedge your bets and buy the 3G model with no service, just in case.
The Memory questions is a little trickier as it really depends on the usage profile of the use. The big memory hogs are photo's, video content, music and online publications that cache content. If you download movies from iTunes (perhaps you travel a lot), figure that a full length film is 4GB, so you will likely want at least a 32GB device just have space enough extra space for movies. Everything else is relatively small. Some publications though are 100MB+ per edition and this can add up. Podcast can take up a fair bit of space also so depending on how many old podcasts you keep on your device, it may be material.
Looking at the usage on my iphone though, I am using 17GB of 32GB. 10GB of that is music, of which I probably only listen to a small subset, but it is nice to have. A few Apps use a lot of Memory (facebook is using 350MB, it must cache a lot of information). I don't download podcasts or any publications or have any movies on the device, so the bulk of the difference is likely photos and videos as I take a lot and do not do a good job of maintaining the library. With iOS5 and photo stream, your photo's end up on multiple devices, so that should be taken into account as well.
A friend recently lost their 16GB iphone and got an 8GB as a replacement and was constantly out of memory. Lesson learned, 8GB is not enough, and apple would seem to agree as they don't even offer an 8GB model any longer. I think that for an iPhone, 16Gb is manageable, but 32GB is better (considering that I am using 17GB currently). 64GB is overkill, at least the way that I use it.
An iPad on the other hand, which is more of a consumption device and may be caching a lot of publications (The new yorker is 150MB per issue and the NY times takes up 60MB) and movies (at 4GB each). If you ever think you will want to put movies on it, and probably even if you don't, I would suggest not getting less that 32GB. Just remember that as time goes on you will always use MORE memory, not less, so perhaps buying more upfront will future proof you a bit.