Hi Ellie
Like you, I only have the 16 GB WiFi iPad and I've been itching to change it for a 3G version. But I held back on my decision to get the 3G version.
Reason why I got this is to test out the ipad and I envisage that I'll be getting a 2nd gen iPad once it is out.
The WiFi version works for me as I'm using an Apple Airport Extreme at home which has an incredible range. To give you an idea of the range, my apartment is on the 8th floor and I'm able to use my iPad at the swimming pool which is situated about 100m (330 ft) away.
Even at that distance, I'm getting 2bars on my iPad wifi signal indicator.
I could probably go further if I bothered to.
So, if you are not on the road frequently, I think the wifi version should suffice.
But if you're really stuck on the idea of upgrading to the 3G version, better do it now while there is still a demand for the wifi version.
Cheers Ellie!!