J-specs are supposed to have about 280bhp stock as apposed to uk specs which have 326bhp, I think in reality most stock j-specs have a similar power output to the uk spec tt if you listen to people who have their cars rolling roaded the j-spec is usually well over 300bhp, They say that the j-spec power difference is a white lie to keep the Japanese car market happy (something to do with emmisions? i think).
I believe the main difference is that the J-spec has the smaller, faster spooling ceramic turbos and the uk spec have the slightly bigger and more laggy steel turbos which can take a bit more boost (upto 1.4bar) than the jspecs, uk specs have 550 injectors and j-specs have 440?
Different people give different opinions but that is what I have read.
If you can find a uk spec with decent service history and low mileage, which is becoming quite difficult that would be the one to go for in my opinion.