ni no kuni has no distinct target demographic, so it didn't sell very well in japan. it will have to depend on sales in the west in order to succeed, especially by adults, especially due to the difficulty (which is easily above xillia). if you mess up in ni no kuni, you will get your ass handed to you. it is actually targeted towards adults, fans of ghibli.
in japan, it's "cool" to like tales. just like it's "cool" to be a group of something. shining, tales, suparobo, street fighter, etc. they're crowds. ni no kuni doesn't have that "backing". people will literally just go out and buy the newest tales without any info on it.
on the other hand, xillia doesn't need western sales, in fact, western OR japanese sales wouldn't have influenced the production of xillia 2 at all. it was already in development during xillia's development, and it's pretty obvious, especially considering how xillia 2's menus, sounds, system, environment, various towns, enemies, and open areas... are copy and pasted from the first game.
oh, and those people who go out and buy tales blind? they're also the ones complaining on 2ch about how xillia 2 has a ton of copy-pasted stuff from the first game, oblivious to the fact that they were made alongside each other.
you're literally trying to compare two different games which rely on two different audiences. tales doesn't need the west, the only reason graces was even localized was because people were badgering their facebook page. it's nice that hideo baba is acknowledging the foreign fanbase, but yeah, it isn't a necessary fanbase. it's additional profit to them. ni no kuni needs the west.
and it will be hailed one of the greatest games ever, because it doesn't fall into the "teenagers save the world!" or "random anime rpg" categories that the mainstream west view tales (and basically all jRPGs) these days. unless you think the western Tales fanbase is larger than the western Studio Ghibli fanbase, in which case, i don't know what to say.
anyway tl;dr: the western release dates have nothing to do with "priority", and one (ni no kuni) is more reliant than the other. in this regard, in actual fact, ni no kuni coming sooner is a good thing.
it was level-5's own advertising that damaged this game, by telling people the PS3 version was in the works, and then releasing the DS version far ahead of time.
this was posted on the 26th of november. you'll see by the western reviews that i was right.