At least 358/2 Days has a plot, Dissidia was a joke and excuse to make a PvP arcade style fighting game while cashing in on FF characters. Days sounds more like Zelda Four Swords, which was incredibly fun, the more people playing, the better. I don't think it's a fair comparison.
I think someone mentioned it, but I think Birth by Sleep looks more like Dissidia (maybe it's the PSP bias?), especially as far as battle style goes. However since BBS doesn't have multiplayer fighting as far as we know, it looks more entertaining. Maybe that was the surreal thing about Dissidia, normally in FF games you don't have complete run around control like in KH games, but since you can it feels strange.
The Famitsu report mentions the game being 35-30 hours long, I'm assuming thats based on the gameplays the reviewers experienced.
dissidia was a Great idea who doesn't want to have there favorite ff characters fight it out? and i don't see how bbs looks anything like dissdia other then graphics but the battle systems are completly diffrent. The only thing dissdia has in common with kh is real time action and using the X and square button for attacks