Traditionally, the iPF8400 has been the better deal of the two (it's often only $1000 or so more expensive, and it comes with at least that much in additional ink - the 8400 comes with full 330ml cartridges, while the 6400 comes with 90 ml starter carts). On the other hand, it is HUGE, and Geraldo's warning is VERY important - once the ink's in it, it only moves flat. The Level 3 ink drain you need to do to tip it in order to get it out uses over $1000 in ink (it drains a total of 1900 ml of ink), two or three maintenance cartridges, and puts a huge strain on internal components. Some people say they've moved these with a Level 2 drain using 600 ml of ink ($300) and a single maintenance cartridge, with no problem, even with the printer briefly on end, but I was actually present at a move (of an 8300) where this was tried, and the printer died. I bet it's luck with exactly which direction it tips unless you do the Level 3. This wasn't a big problem with earlier 6000 series models, but I've read that the 6400, as it got the bigger ink tanks, also picked up the sub-tank system that makes the 8000 series printers such a pain to move! Of course, a 6400 is only about 4 feet wide and something like 120 lbs, so you may be able to move it flat where the much larger 8400 would have to tilt. Remember that you can tip an 8400 (including on end) to get it IN the room it's going to - it's only once the cartridges are loaded that it's permanently installed.
The other negative feature of the 8000 series is that they ARE front-loading, but they load in a complex paper path from the BOTTOM of the printer. I much prefer the top front loading of the 6000 series. I can see why Canon did this - the 8400 would be very hard to load in a room with low ceilings if it loaded like a 6400, due to the very long spindles - but I really don't like the kneeling or squatting under the printer with a heavy roll. The best loading system of all is the current Epsons, with their beautiful spindleless top loading, which requires no extra space at all, but Epsons, of course mean more care with clogging - I presently have a (brand-new) Epson 7900, and it has NOT given me any initial problems (one of the times they are most likely to clog is actually right after installation). Fingers crossed, and I start each day with a print, which is a recommended way of keeping Epsons happy.
The most troublefree big printer I've used, other than my week-old 7900, was an iPF6100. The 6000 series are great machines - but the pricing on the 8400 sure is tempting.
Something else to consider is canvas... If you aren't doing canvas, a 24" printer is darned big, but canvas prints go bigger all the time (both because of the extra required for the gallery wrap and because there is no mat or frame, so a larger print fits in the same space.
