Superphreak is correct if you anticipate using a lot of space if not then the 4g / wifi would be my recommendation. This is my 2nd iPad and 3rd I phone and I have never utilized more than 16 gb. Remember there are others that utilize a great deal of gb so analyze what you are going to be doing. I purchased the 1st iPad and then the 3rd iteration. The speed difference between 3g and 4g LTE is significant to me and the wifi is great. Hope this helps.
It depends on your usage patterns. I lean towards storage vs availability of mobile on the go. During the first year of my iPad 2 3G ownership, I used the 3G zero times. I used the heck out of the 64GB of storage though.
I'd go with the larger size. You have to remember that the apps designed for the new display are going to be larger. Not to mention taking any video at 1080p will likely be a large file. I really can't see how 16G will be enough for anyone. Keep in mind that BestBuy has a 1 month return policy so if you did get the 16G and noticed it wasn't going to be large enough you could always exchange it.
I'm in the size over 4G capability group. I actually decided to get a 64 gb iPad 3 from a 32 gb iPad 1. Personally, I only had ONE time over three years that I ever wanted to have the ability to have 3G on my iPad 1 (didn't need it). However, I don't travel much, and when I do, I'm an AT&T subscriber so I have access to MANY wi-fi hotspots. I also have my iPhone 4S jailbroken, so in the FEW instances that I am away from wi-fi and I can use MyWi. I've never needed the 3G/4G. (That and I don't want to pay for multiple data streams...my iPhone is enough!)
Now...if AT&T adopts a data sharing model...I may have wished I had a 4G iPad 3! But I think I'll be fine.