It's hard to tell but as a Londoner, I dont have easy access to any decent hills so when I trained for UTMB in 2007 I supplemented running up and down hampstead heath with repeats of the 8 flights of stairs in my apartment block and lunges in the gym. Who knows what made the difference but I managed under 29 hours.
My personal opinion is if you have access to hills and you want to improve your ability to handle running down hills then you should run down hills, however one or two sessions of lunges a week only takes 20-30 minutes and can significantly improve the eccentric ability of your quads not to mention increase the strength in your glutes and improve your overall running efficiency.
Dynamic lunges are even better than normal lunges.