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I'd say either is gonna be a big improvement over the 'C' code. I dunno if you'd be able to tell much of a difference between the two 'J' codes though.
One of the off-road mags did a test a number of years ago and there is a difference, not that I can remember which was better . I'm pretty sure though any difference is only seen on the dyno, would be hard to tell by seat of the pants. Either would be a good improvement over the C code though.