They are identical. They have the same performance. Skunk2 was made to fit the GSR, the ITR IM was not, it requires work.

Get the Skunk2 if you are going to get one, cheaper and similar construction.

I personally would stay with the gsr IM, a really neat piece of engineering. Tuned right (and with the right headwork), you can get some nasty midrange, IMO.