Thanks to the wonders of streaming I've compared the Nightingale first volume to the Kontra in the same works (quartets 1, 3 and 15).
It's been an interesting exercise. I wouldn't say there are many enormous interpretative differences. My impression are the Kontra are a little sharper edged, and the Nightingale have a slightly warmer and more varied palette.
Certainly there were moments where I thought that the Nightingale were allowing me to hear the music a bit differently, with just a fraction more colour and shape (helped of course by being very nicely recorded), and I guess a lyric quality. Against that are certain moments where I just wasn't as convinced by what they were doing. So, for example, while I found the Nightingale to have lots of nice moments in the first 2 movements of SQ no.1, their finale was a bit disappointing. It's a sort of perpetual motion whereas the Kontra make it a much more pointed 5/4 dance. This was one of the most specific differences between the two ensembles and it's one where I definitely came down on the Kontra side.
Similarly in SQ no.15, there are some really nice moments from the Nightingale, but somehow the 2nd movement just sort of passed me by. As for SQ no.3, well, I guess this exercise demonstrated to me that it's not one of my favourite works anyway, as neither performance enthused me more than the other.
So I'm not feeling a clear 'winner' here. Are the Kontra good? Yes, I generally think so. Are the Nightingale good? Yes, so far I think they're generally pretty good. Not better, just different. Some definite strengths, but sometimes not preferable.