D600 mount and lens question

1) as long you always hold the lens under with your left hand [by standard practice] you shouldn't worry too much. heck i had even mounted the 24-70 on a D90 before. No biggie!
2) I haven't used the 24-120 f4 so I can only give my short view on the 24-70mm. Excellent throughout the range, nice colors + contrast! For me it is a good walkaround lens [in my definition]...

but if you want more range then obviously 24-120 f4. Obviously u know the limitations that this is f/4 constant so if in low light f/4 may still be hard to use in certain cases but in daylight either lens is ok.