I got the D3 and I'm really happy about it. I wanted fastness and good iso (which I think D3 has, but I havent tried D600), something that was built solid (I'm a bit clumsy and I often get my camera dirty) and then I didn't really want movie or flash - flash I would like to get separately.
Its a little heavy yes, but I got used to it really fast and now our old camera seems SOOO tiny, lol. Im a standard slim girl, and I dont really mind walking around with this D3.
Im glad the files dont take up to much space, as I remember D600 is much larger, is that right? I am horrible at deleting pictures, for some reason I save way to many of them.
One thing is the lack of self cleaning sensor, I would have loved that, but oh well.
And I agree with Eye-Level. I still make loads of bad pictures but I get good ones too - but only because I've been photographing for several years. Its not just click and hurray because my camera are a pro, sometimes I think that the more I learn, the harder it gets. But you probably know that.
Oh and I love the full frame