j-pop is more different to american music than k-pop, idol j-pop songs are absolutely different to what's pop in america, just listen to any j-pop idol or idol group and you'll see how different it is. same with their fashion, it's ussually quite weird (not everyone though). regular japanese singers' pop is more similar, but they're still different vocally (of course the language make them use their voices in a different way) so the sound is still different (lol or maybe it's just me), while k-pop singers or idols have similar entonation to american ones and their music is more similar (except when they go for the so called 'cute concept'), and while the fashion the k-pop idols wear is crazy too, it's overall more trendy and more cool, plus k-pop singers ussually have great choreographies, (I've seen j-pop groups with amazing dancers, like Flower, E-Girls, Happiness, etc. but their choreographies weren't as good as k-pop ones, hard choreographies, but not as creative, and for the rest of groups or solos their dancing is not hard or creative).
I (as a person who grew up with western music, and actually enjoys western music, including american artists) like k-pop more, I tried to get into j-pop but couldn't :/ their idol pop has good melodies but that strage sound I can't enjoy and non-idol pop is.. kind of.. plain, not entertaining, in my humble opinion. I can like it but not love it or get obsessed with it.
on the other hand, j-rock is great, I really love Asian Kung-Fu Generation, frumpool, NICO Touches The Walls and YUI, while I can't get to like korean rock bands, there are very few korean rock bands though, but they didn't amaze me. The only pop artist (but mostly does ballads) I absolutely love of japan is Utada Hikaru (Hikki) she's amazing, I met her through Kindom Hearts and she's been one of my favorite artists ever since, but she's on a hiatus :( btw, something that I love about j-pop idols is that they are hilarious and adorable, I've watched AKB48 and Johnny groups shows lots of times even if I don't enjoy their music because of how great their personalities are, they're great at variety shows, and lots of them can seriously act. (btw, Johnny & Associates is a j-pop company that produces only boybands and has most of the most popular boybands of the country, in case you didn't know hwat I meant with Johnny groups ^^)
anyways, the point is I personally prefer k-pop, I always speak too much xD