I have used both, strictly speaking I would say the kiss x4 is a little better, even though I'm a Nikon user myself. I find it amazing when taking photo at night.
But honestly speaking the difference may be so small that once you started shooting (instead of just playing with the camera), it's more or less the same.
You should try out both of them at a shop. I used to say Canon feels very bad in my hand, especially with the x2 and x3, but x4 is different, it feels very good.
Are you in Japan? Since you call it the kiss x4 instead of 550D or a rebel. Not sure about price there but over at where I live the kiss x4 does give a bit better value for money over the D90.
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