Reading the replies posted so far, the consensus seems to be "go for the max", you can't up grade later and you'll never know what you need in the future...
I wanted to offer a different perspective based on my personal experience. I have an expense account, and so I went for "the ultimate" (64 GB with 3G) when purchasing my 1st iPad a year ago. So how did I like it in the past year...
First of all, I was never closed to filling up that 64GB. Part of the reason is I don't have THAT much music, apps and movies. The other part of the reason was my sync computer being a 128 GB MacBook Air. Don't forget that if you fill your iPad with 64 GB of data, it backs up to your computer taking the same amount of data storage. You may wonder why I chose to use my MBA, instead of a desktop, as my sync computer. This is because I want to be able to sync and update my iPad when I travel on business, You never know what you'll need.
Now fast forward to present time, iPad 2 time. It was time to sell my iPad 1, I went with kijiji (like craiglist). I asked for $500, and it took close to 2 weeks before it's sold, which tells me that my price was reasonable, not too low, not too high. FYI, the 16GB base model is selling around $350 to 400. My loss, dollar-wise, was definitely much higher compared to those selling the cheaper models. Also, I feel that it is generally more difficult to sell a high end used iPad than the base model. People buying used generally want to sell money, and tend to go for the cheapest, caring less about the specs.
So finally, time to choose my new iPad 2. I did some research and came across this article, which states that the resale value of the 32 and 64 GB is only $50 and 75 more than the 16 GB model. For 3G, it'll only add $25 to the resale value:
http://gigaom.com/apple/which-ipad-2-should-you-buy/
Even though with my expense account, I still want to use it efficiently and save some for an iPhone 5

I ended up going with the 16 GB model, for obvious reasons. I did opt in for the 3G, because in the past 8 months, my 3G iPad really made my life much easier in 2 occasions. Even just for the 2 occasions, it was worth it in my opinion. I could always delete something on my iPad to make room if I really need to, but I can't add 3G.