The 2400 makes excellent color prints out of the box, with quite good profiles available on Epson's website. The 1800 reputedly has somewhat better saturation for neon colors and provides a "gloss optimizer" to minimize bronzing, but these features aren't very relevant for portraits. The 2400 will print happily on matte or fine art papers and as noted also produces really excellent black & white prints with the native Epson driver.
On the downside, the 2400 burns through ink cartridges at about US $14 retail each, and the ink costs become a killer if you're making a lot of large prints. In a production environment the much lower ink costs for the Epson 4800 will quickly cover the 3x higher purchase price.
Ditto the above. I had the 2200 and just upgraded to the 2400 about 5 months ago. The 2400 makes great prints right out of the box printing directly from Photoshop CS2.
I just upgrade to Image Print v6.1 and now I can use this to print to my 2400. The results are slightly better than the native Epson drivers with more detail in the shadows.
You can't go wrong with Epson printers.