For practical use, the Snow models are much better. Get the fastest one you can, and load it up with RAM.
The 800mhz model will get you a Radeon 7500 GPU. If you're buying a 14", you get 256mb of RAM soldered to the board, which makes the maximum ram 768mb(256+512). The 12" are stuck with 640mb(256+512). The best Clamshell has 64mb soldered to the board, which leaves you at 576mb. Some first generations only had 32mb onboard, which gets you stuck at 544mb.
The Clamshells are a neat design for a collector, but have quite a few downsides for use now. The 800x600 screen will feel very cramped on the modern web(I'm amazed now how I was ever able to stand it "back in the day" when it was standard even on desktops). The Snow models grow to 1024x768, which is still a bit small my modern standards but still wide enough for most web pages. All but the last generation Clamshells lack any high speed I/O, as they are limited to USB 1.1 and don't have Firewire. All the Snow models do get Firewire, although still only USB 1.1.
There's a certain charm about even the Snow model iBooks and I have one, but when it comes down to it I still prefer a Pismo. They are a little bit larger and heavier, but can hold a lot more RAM(up to 1gb) and are much easier to work on. They also have a PCMIA slot, so under Tiger with the "right" PCMIA WiFi card(the ones I have are Belkin-branded, although I don't know the model number) can connect to modern WPA2 networks with modern encryption schemes(although at the cost of any WiFi under OS 9). You could also put in a USB 2.0 PCMIA card or any other PCMIA-compatible peripheral you desire. The biggest downside to Pismos-at least as I see it(aside from the weight) is that aftermarket batteries are expensive.
