At low compression, there usually isn't a lot of difference between a JPG and a JP2 (unless you have a lot of red in the pic, JPG is terrible with red). At high compression however, JP2 is quite a bit better. Here's an example:
The one on the left is a 9k JPG. The one in the middle is a 9k JP2. The one on the right is the original pic.
Jpeg vs JP2k comparison
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