Constant f/4 or VR @300mm?

Okay, question….

Looking to upgrade the long end of my 18-200VR for a telephoto zoom. My choice is currently between the Nikkor 70-300mmVR & Sigma 100-300mm f4…

With the 70-300 getting (mostly) good feedback and the Sigma getting good reviews (Photozone), my question is which is more desirable the constant f4 or the VR @ 300mm?

(or another way of asking - Is the 'f4' worth x2 the £££'s of 'VR'?)