I'm trying to decide between the Canon iP8500 and the Epson R800, but I haven't been able to find much comparing the two directly online. I'm getting the printer primarily to print DVD labels/covers, so I'd be most concerned about the quality of the disc printing, and possibly the per-disc cost, if the difference is significant. Can anyone help?

Thanks!

If printing cost is a concern then I suggest getting the Canon iP8500. It uses the older, non-chipped, BCI-6 cartridges which can be bought with compatible ink for less than $2 a cartridge. I buy BCI-6 cartridges from here: https://www.neximaging.com I have not had any issues with the quality of their cartridges or the third party inks they use in them.

I can't speak for Cannon because I have an R800, excellent piece of equipment.
I also have an Epson R380 and the R800 gets way better ink mileage.
The quality of both printers is excellent.

If disc printing is the only concern, any of the Canon models will suffice. Although I also would prefer the older models with non-chipped tanks, the chipped tanks are not very difficult to get around. Point being that the IP8500 might be overkill for just printing discs.

i've a ip5200 and the compatable carts don't work with it, the cheapest original in will cost me £40 approx, so i've bought a R265 for just over what a set of canon carts are, i just need to test it.

Yes, compatible tanks will work fine with it, as long as it's the right ink. You simply need to transfer the chip to the new tanks and disable the low ink warnings. Instructions are available at ink sellers online.

Instructions for chips

Recommended compatible tank