I have both and they're both excellent, rivalring prime lenses. The 35-70 might be a bit better optically, but not enough to compensate for the extra convenience and range of the N lens converted to EOS mount. I use the 24-85 N for 70% of my landscape shooting on my 1Ds III and love the results. Do keep in mind I shoot mostly at f16. My 2 cents.
DJ