As many have said, depends on your personal preference.
My first Rotring 600 was a Series 1 (gnurled grip... all my Rotrings are Series 1) in silver, with the 18k gold EF nib. I love that thing to bits. I ended up getting silver because I wanted the 18k EF nib which wasn't available at the time in the black finish.
Then, I thought, well.. since I like this one so much, why not get a black one, even if with a different nib? So, I did, with an OM nib.
Then I found out that they made matching mechanical pencils! I needed something a little sturdier that was also a change from the seafoam green Sanford Logo IV I'd been carrying around for a decade. And I found it in the Rotring 600 mechanical pencil... which I picked up in black, along with the tikki eraser thing...
I'm telling you, either I have mild OCD or Rotrings are just addicting. I recently found a 2mm lead holder which I had shipped from Japan, just so I could use it as my primary sketching pencil. I thought my Rotring 600 MP was the end all, be all too... until I found the Rotring 800 MP