I purchased my iPad on Sep 6, 2010, and could only be happier if it did allow the use of Flash for animations on websites - but there is another topic area for that discussion. Yes, I knew beforehand that Flash is not currently allowed, but You Tube videos do play just fine.
Now to get back to your question - after several days of intensive analysis, I decided to get the 64 GB with the 3G capability (they all have wi-fi), because I remember times when I was traveling (within the U.S.) and needed to access the internet in spots where I had no wi-fi access. At the time, my smartphone was not capable of being a wi-fi hotspot, so I was totally left out of accessing what I needed in the location I found myself.
The first two weeks I had my wonderful iPad, I did not set up my 3G account, but this past weekend I did because I knew I was going to be traveling in spots with no wi-fi access (wide-open countryside). I found it to be absolutely worth the extra $s to have the 3G capability, and it works wonderfully well. I did, however, first check the coverage area for the area I would be traveling. You are not limited to AT&T for the 3G service either.
The good thing about the AT&T 3G service for the iPad is that it is not a long-term commitment - if you find that you are accessing sites that create a lot of heavy data usage, 2 GB can be used up pretty quickly and then the cost can become prohibitive, but not to worry - you can opt out of automatic renewal each month or when you have reached the bottom of the data pot. The best thing to do is to check other carriers' plans first, but I wanted to try the AT&T 3G service to see how much data I was actually using (a ton on some apps). I am probably going to activate the Sprint Wi-fi Hot Spot option on my smartphone and pay a tiny bit more for scads more data usage.
Another point to consider is that if your smartphone is with another carrier, adding the AT&T 3G service for iPad can increase the likelihood of actually being able to get 3G service without roaming charges (depending on where you are at the time).
There is no getting around doing your homework before jumping into the wonderful world of iPad so that you can use your technology dollars wisely.
I am happy I chose the 64 GB, wi-fi + 3G model iPad, and I would do it all over again if I had to.
Bottom line: Whichever model you choose, enjoy it to its fullest capabilities - just learn as much as you can beforehand without getting mired in indecisiveness.