Since I have both of these, I thought I'd write up a bit of a comparison.
Now they have alot in common - they have a nearly identical form factor. They're the same color and share digitizer hardware (the same pen will work in both) and overall port placement.
How do they differ? First off, price: I paid roughly 10 times more for the 2300 than the 1200 around a year ago. Which makes the 1200 a clear bargain. But Fujitsu clearly put in alot of thought and design when they upped the model number. They 'bout doubled the processor speed and still have the same battery life. But if I had to place the bulk of the changes into a single description, it'd be "more complete". You don't need the port replicator because so much is already there. Stand-alone, the 2300 has a USB port, a floppy port (behind the battery}, keyboard and mouse ports. It also has a built-in 56k modem and the digitizer is recognized as a plug and play device. The display on my 2300 is an 800x600TFT whereas the 1200 has a 640x480 DSTN (albiet a nice one...}
Well, that's all I can think of right now.