Actually the addressable dpi in the case of Seiko is 720 dpi. So horizontally you will always have 720 dpi. Vertically (i.e. along the media movement) you can have 360 dpi in 2 pass, 720 dpi in 4 pass and something like 1440 in 8 pass. That's my guess. So actually the 2 pass mode is 720 x 360 dpi and there is no difference between this and the 720 x 720 dpi mode as far as the droplet size is concerned - it is the same! But the spacing is different. That is why you'd have to really try hard to come up with a fully saturated, deeply coloured profile for 720 x 360 dpi. So yes, apart from slightly lighter printout and visible banding caused by such a wide bidirectional print (some media are forgiving, some are not - try 2 pass on cast vinyl), you should not see any difference in quality. The horizontal dpi and head speed are the same.
On the other hand... my Seiko is only coming here next week ;)
- less ink used? Yes, that's my guess. Don't know how much less tho. You can expect 10 to 15% less ink usage on some works when using 4 colours instead of 6, but that will depend on the content of the image. If it is full of light blues and light magentas, then surely you will not only feel less ink used, but of course also see the much worse quality than in case of six colours.
- faster RIP time? Nah... I don't know. But i've seen many times that actually even tho the ripping in the lower quality modes was faster, the computers didn't manage to send that much info to the printer. It happened a few times in my company, it happened during some demos i've seen. And actually such choking is deadly for the print because if there is nonuniform velocity of media feed, then it is also nonuniformly heated! And that's just bad.
