>Other than lighter weight, built in GPS and WiFi the D600 >beats the 6D in nearly every way. > It should be awhile before my EOS 6D and Sony SLT-A99 arrive, but judging from my 5D Mark III and (admitedly APS-C, not full-frame)Sony A77, my D600 should out-do them both in the high ISO *with detail* competition. My 5D Mark III looks fairly smooth at ISO 3200 and above, but the noise is mushed up, thanks to non-optional default NR applied to everything. I expect the 6D to have a similar plasticky quality, and I'll prefer the D600's better detail at high ISOs. You can always apply extra NR to RAW files, but can't remove mushiness that's added by default.
My Sony cameras have always been less good at higher ISOs, dating back to the DSLR-A850, although there have been improvements since then, making the latest SLT models more acceptable. Of course, losing a third of a stop to the semi-transparent mirror means you need to up the ISO a notch anyway.
My all-time favorite ISO champ was the D3s, and I don't regret trading it in on the D4 in that regard. The thing I like about Sony's APS-C models is focus peaking in manual focus mode (it really makes manual focus a pleasure for my old eyes in the studio under relatively dim modeling lights), and the ability to use cheap-but-good Minolta A-mount lenses. And for that, I've ended up using the NEX-7 (with E- to A- adapter) more than SLT models.
High ISO, ergonomics, faster autofocus (at least vs. the EOS 6D; the 5D Mark III is pretty good) give the D600 a fairly substantial edge over the competition in the same price rainge.