I had a 2400 and an 1800 when I bought the 3800. I hardly use the 2400 and 1800 now (the 1800 prints CDs and DVDs, 'cause the 2400 and 3800 can't).
The 3800 runs a LOT longer on its ink cartridges (as it ought!), and my estimate is that it costs about half the price in ink to run. The cartridges aren't cheap, but they do last a long time. Not like the "change a cartridge every few prints" exercise on the 2400 and 1800.
The 3800 can print A2, the others can't.
The 3800 switches blacks automagically (and cheaply, wasting very little ink).
The 3800 print quality is the same as the 2400 - uses the exact same inks, at the same resolution, so you'd hope so!
I have a feeling that the 3800 feeds cut sheet paper a little better that the 2400/1800 - can't say that for certain, but I feel it's easier / less fuss to load.
The 3800 does not take roll feed, and I have NOT managed to print on canvas using it - the canvas just won't feed properly. This is the only problem I've had with it. Maybe you should keep the 2400 for printing canvas? I was trying to print on Canson canvas, which fairly light - maybe other canvas would work better?
they say you have to take a thousand bad photos before you can take a good one - I have mastered the bad photos part...