I have the game on both systems, but I lost my DS one, and when I saw this for the PSP I was like "Hmmm, ok!"
So, I played both games (still in the beginning of the PSP version) and I can't complain much. I don't really see the DS one being superior to the PSP, except for maybe the battle setup where you have to cycle through everything to see your allies' or enemies' movements or to equip key items or weapons. The wisp movement isn't horendous as one player made it out to be; I have yet to adjust the speed. I didn't go too far into the DS one, but I don't think the gameplay could have changed that much.
If you only have a PSP, don't bother getting a DS just for this game. I actually don't like SRPGs very much, but this one captures my attention for some reason. Could be the Norse mythology theme (love VP).
I've played Covenant of the Plume, Luminous Arc 1, FFTactics (forget which one, not the original though), but KitN is pretty similar and different at the same time. The space, for one, is very, very restrictive, and movement is all but impossible. The most important movement, though, is the wisp, and not so much the characters themselves. It's way more interactive than other SRPGs, and requires good maneuvering on your part.
So, if you like to just sit back and command your characters with peace of mind, then this game is not for you. If you're looking for a new twist on the SRPG genre, and don't mind being a hell of a lot more active in-game, then this might just be up your alley.