Yes, this gets debated a lot on here.
I've owned the Epson 7900 and now own the Canon 8400. There are advantages to both and from a print quality standpoint, I would say that you would be happy with both. A quick summary of pros and cons for me without getting into arguments about color gamut:
-Outstandingly fine dither (though this can really only be seen with a loupe-- but it is remarkable).
-Straight paper path for printing on thick materials, posterboard, metal, etc.
-Slightly faster to change rolls with no spindle and a very clever 2 inch to 3 inch core switch.
-In theory, an easier cut sheet feed.
-Incredible ease and speed of operation:
-no PK/MK switching and the extra cleaning cycle/time waste
-no wasted ink/time with cleaning cycles in general.
-less time "thinking." It just prints and is very low fuss.
-Comes with full-sized ink tanks on the purchase (the 6400 does not). If you buy it with a good rebate going on, it's really a great deal
I did not think I cared as much as I do about operational ease and efficiency, but it is remarkable how much less time I waste (and ink) while printing-- not running cleaning cycles, not having to switch between mk and pk (which I thought I didn't care about, but the ease and speed of going back and forth between papers is really nice). Also, I have the sheet feed down and it's very easy and fast for me with 100% success of getting sheets loaded perfectly straight.
My 7900 made great prints. I had some early problems with it and had D1 out. And there were a few lingering oddities, but by and large I had a year and a half of problem free printing though a reasonable amount of ink wastage with cleaning and pk/mk switching. Then, for quite a while, I was running daily cleaning cycles-- sometimes running cleaning cycles in the morning, printing, and then having a clog by noon. Some people run these printers without problem and then they are great. While I know that things like humidity and conditions, cutting a lot of matte papers, and frequency of use play a role, I do think there is some random outcome to printhead failure and when I had the permanent clogs, it was cheaper to buy a new printer than have D1 fix it. I did not find Epson particularly helpful at this stage.
So far so good with the Canon, but objectively, I've only had it for three months and would have said not quite the same thing with my Epson in the first three months-- but I hadn't had the major problems yet. Time will tell, but at least the Canon has replaceable heads.
Finally, I took a good look at Canon two generations ago and then, for me Epson was still clearly ahead. I didn't really look at the x300 series, but when I looked the x400, it was the first time that I would have considered Canon. I had four Epsons in the past and this is my first Canon and I like it. And competition is good. . . .