I say easily the base WiFi model. I'm going to get a 32GB iPad WiFi, but I'm not kidding myself that the last $100 isn't as good of a value as the first $500. It's the same deal with my 32GB iPhone. I know that 16GB of flash memory doesn't cost Apple anywhere near $100. Why? Because if it did, the 64GB iPad would cost $800. You can go buy a 32GB flash drive for less than $50. Apple buys flash memory in insane bulk numbers. If the numbers were $499, $549 and $599, I'd feel more like I was getting my money's worth on the top end.
Alas, that's what the prices are. We're willing to pay that much for the extra storage, so Apple keeps it that way.
The 3G iPads don't really seem worth it to me at all. If I knew I could use the GPS without using any data plan, then maybe. With the AT&T announcement about using the iPhone as a wireless hotspot, I have zero need for a 3G connection on just my iPad. I can get a dongle that connects to the dock connector to give me GPS for $99. I'm mainly thinking about the Star Walk app, but maybe something else comes up that I would use GPS for. Either way, I don't need to fork over the $130 for the 3G radio when I've already got that in my iPhone, which is on me at all times. I'd rather spend that money on the extra storage and a smart cover.