People seem to be making a lot of assumptions about other people's uses. Here are a couple items to consider about 3G:
1) How often will 3G be needed? It's not overly complicated, but can be a pain to get on your phone, turn on the hotspot, connect the iPad, THEN do what you need to do. If you do this several times a day, it's a nuisance. With 3G on the iPad, it's there when you need it.
2) How long will you need 3G. Hotspot drains the phone and doesn't last 4 hours. 3G on the iPad lasts as long as the iPad is charged. No need to worry about finding a plug for your phone as you've drained the battery tethering and can't even using it for a phone as it's got no power. If you are near a plug, most likely you are near WiFi.
3) How much mail do you have? Every time you connect, you have to download your mail that's waiting which means slow internet speeds doing what you were trying to do in the first place. This can be fixed by putting mail in manual mode, but again, another item you have to change depending on where you are.
As far as space, it really depends on what you want. Games can be huge. Among those already listed (Rage, Infinity Blade, Hot Pursuit, GTA being over 3GB), you also have Splinter Cell at 523MB, Spiderman at 492MB, Lego Harry Potter at 454MB, Real Racing 2 at 403MB, etc. I have 22 applications over 200MB, 16 of those are over 325MB. I don't think it's been mentioned that with some if not most, you need double the space free for upgrades, sometimes more. I personally don't add pictures to my iPad as I don't have a need to look at 4GB of pictures if I don't have my computer. I have a very small subset of music on it. Not for daily use, but for trips so I don't have to have both the iPad and iPhone out on a flight. If I listen to music, normally it's on the iPhone. I have several videos. Some for the kids if we are out, some for me on trips. Most could be eliminated if necessary.
With all that said, there is no way I could work with a 16GB. I tried, but in reality, it only has around 14GB. Add a few apps and just one or two videos and your full. Every time I wanted to do anything, I had to go through the iPad to figure out what I wanted to remove. I could probably be fine with 32GB, but I went ahead with the 64GB just to be safe. I would not depend on jailbreaking as for one, it's not out yet on the iPad2. Also, you have to have the adapter and usb stick which is both awkward and can make it difficult to hold in landscape.
For the 3G, I ended up getting it. My original iPad was WiFi with a MiFi for connectivity. It worked, but wasn't convenient. I feel it's much more productive with the built in 3G for the reasons I mentioned above and I use the iPad much more than I did before.
In my opinion and my situation, spend the extra $100 and get both. You are already spending $600, make it much more convenient and pony up the extra cash.