As Daring Fireball notes:
iPad’s Safari isn’t able to keep nearly as many pages open as I can on my 3GS. In fact, sometimes it seems I can only have one, and every page I switch to gets completely reloaded.
This is a serious limitation that affects the performance of Safari in particular, but of the iPad as a whole.
Reloading pages in Safari is annoying and, as John Gruber notes, can cause data loss in cases where you have unsubmitted data on a page that gets dumped.
On WiFi this is a serious problem, but on 3G it is going to be a disaster! I predict a huge customer backlash from the performance problems unless the 3G version of the iPad has more RAM.
It's possible that the $129 difference between the two models includes additional RAM, though I would think it's unlikely.
Imagine for a minute, though, that it does have more RAM. I think that would result in a different, but also huge, customer backlash from those that bought the WiFi version! People will feel like Apple once again offered a better value a short while after introducing a new product.
Either way I think the 3G performance on the iPad is going to cause some consternation.