It is largely debatable as to say which is "better," even with the fact that both have a mid-centric signature. The GR06 had better extension in the highs (consequently, it has better sparkle too) and a more full low end and impact than the hollower SE215. Both had a very full and embodied, warmish midrange, However, for mids, I do side with the SE215, which I found sounding smoother and better defined than the GR06. While I found the GR06 having better extension, yet putting the mid forward, the SE215 is more balanced in placing the mids forward.
If I had to choose, I found the GR06 a tad better than the SE215. But only slightly, as I really do love the SE215 midrange.