Go for the SB-700. I love the 2 little dedicated slider switches for mode (TTL, M or GN) and the beam width selector. The selector wheel is pretty cool too. Makes it easy to navigate menus or change flash power. Whatever you do, make sure you learn how to use your speedlight with the CLS built into the D600. My only gripe with the CLS is the optical window on the speedlight needs to "see" the trigger flash which is usually just the pop-up flash set to CMD mode (custom setting e3). THe SB-700 also supports FP (focal plane) sync, which means you can use your flash at shutter speeds faster than 1/250. Yes, you lose a lot of power when you use FP sync, but it's a great option that some of the older speedlights don't have. Also, when you do get your flash off the camera - make sure it's properly attached to something or otherwise strapped down to something secure.