They are both good machines. Yesterday I attended a large format printing workshop at Canon's Irvine HQ that was sponsored by Canon and LexJet. They had both machines on display and up and running for the workshop. At the end of the day, it just might come down to how much room you have (the 8400 is BIG) and whether you need the extra width provided by the 8400. I have an 8300 and I have been pretty happy with it, other than 1 major hiccup that I will discuss below.
I do quite a bit of large format canvas work (using BC Lyve) and for a show of my photographic work in Italy next month, (
www.visionsfromlarmorra.com), all the prints were done large format ... 24 x 36 on LexJet Sunset Photo Metallic Paper. Nothing but excellent quality from the 8300 on both canvas and photo paper.
As other have said: these machines have to be used regularly and/or left powered 24/7. It doesn't say this anywhere in the printed or on-line manuals. I turned my machine off when not in use and there were times when I didn't print for several weeks. 3 months out of warranty, it stopped working with a bizarre error code. From what I could decipher from the code, I thought it was the heads. I called Canon, they had me do a status check over the phone (that confirmed very little ink had actually been run through the heads). However, the error code seemed to indicate it was actually something more than the heads .... a small logic board. Since I was out of warranty, the "service call" by Pitney Bowes (they do the service for Canon) was $1,500.00 + tax. They replaced the small board, put in new heads, and changed the maintenance cartridge. Oh, all of that also used up about 1/2 of the ink in the tanks .... so I also ended up having to buy more ink! When I called Canon support, the first question their service tech asked was, " do you turn your machine off." When I said, "Yes," his reply was, "don't ever do that." (However, with their new firmware, in order to receive Energy Star compliance by default the machine will shut-off automatically on its own if not being used for a period of time .... which can be changed if you drill down into the firmware's menu and change that default.)
So, a word to the wise, consider buying the maintenance contract, (it is extremely hard to find third party maintenance/repair companies to work on Canon machines since Canon keeps a tight control on parts), and, if nothing else, leave your machine powered 24/7.
With all of that said, I really don't think you will be disappointed with either machine.
Good luck.