The 3800 is definitely the go - it is much cheaper to run in terms of ink, and turns out prints that look better to my eyes than the R2400. I had an R800, and currently have the R1800, R2400 and the 3800. Although I got the 3800 mainly for the ability to print A2-size (which I have done - the results are excellent), I now use it for the vast majority of my printing because I have to change ink cartridges so rarely.
I do use the R1800 occasionally, but only for printing CDs or DVDs (the 2400 and 3800 won't print those).
The 2400 has been relegated to printing model releases mostly
