Okay, these are the apps I intend to store on my iPad (in addition to the default ones). I looked up the sizes on iTunes.
WeatherEye HD iPad - 7.7MB
iPeriod (laugh away...
) - ~39MB
Calendar Tracker - 0.9MB
Countdown - 4.6MB
What's On? TV - 11.6MB
Twitter - 10.2MB
Tumblr - 7.5MB
Words With Friends HD - 23.6MB
Emoji - 1.6MB
Bejewelled Blitz - 86MB if HD
Photoshop Express - 34.2MB
Photo Collage - 17.7Mb
Houzz - 6.4MB
Angry Birds HD - 45.4MB
Kindle - ~25MB
Kobo - ~25MB
That comes to 343MB... I know that over time I will probably add more apps, which will take more space and that apps will likely get larger and more complex. Plus, I will buy ebooks to read with the Kindle and Kobo apps (but I think these are small? Around 2 MB?)
I'll probably add some photos (though not take photos with the iPad itself, or at least very rarely).
I use the iCal and Mail extensively (via MS Exchange) but I don't know how much space my multiple calendars and two email accounts worth of stuff takes up? I'm guessing it's not that extensive? Given that even with 3.3GB of music (which I would not put on my iPad) and many more apps on my iPhone I'm only using 9.6 GB on my iPhone, lol.
Given this, I think 16 GB would suffice? Even if I add a movie from time to time when traveling (rare; I don't even have videos on my iPhone), it seems I'd have enough space for everything? I plan to keep this for at least 2 years so I don't want to get to a point where I have to manage my media extensively if and when apps get larger but it's really seeming like this will suffice, since the most gaming I do is Words With Friends and maaaybe Angry Birds. lol.