Loz, I suspect Stubbsy is right about costs. I guessed my R310 was about a $1 per A4, the R1800 (identical to the R800 inside) is much better, but how much I don't know. Have a look for an online comparison as they will at least use the same methodology and print to compare (important as black is cheaper on the Epson, handy for my moon shots

).
Like Steve, my decision wasn't based on print costs as there is no clear winner here, but on longevity of prints. My thoughts were that if I was going to pay relatively big money for a printer (an R210 can be had for a bit over $200 and gives great A4 prints, my R1800 was over $1k), I wanted to be able to give the prints to people and not have them fade. So I went for what I thought was the best solution, not the most cost effective as the pricing is so close.