70-200mm f/4L IS @ Wide MTF:
Based on MTF chart given by Canon, 70-200 f/4L IS and 70-300L is VERY VERY close in quality. Having used both lens before, I'm in agreement with this. My personal view on the 2 lens are:
70-200mm f/4L IS
+ Constant aperture
+ Does not protrude when zooming
+ Small sized and light weight, compatible with 1.4x extender for non EOS-1D bodies and compatible with 2x extender with EOS-1D bodies. If your 1D is the APS-H sensor, you can get a 364mm f/5.6 or 520mm f/8 and retains AF, at a affordable price with portability.
- Too thin, I don't like the "grip" feel of the lens
70-300L
+ Extra 100mm in focal length with great optics as it's 70-200mm brothers
+ Thicker barrel, giving a very solid grip and feel.
+ Shorter than the 70-200mm brothers, which means, more portability and easier packing.
+ Not as common in 2nd hand market, which means you don't face the problem of "oversupply" and having to lower price to stay competitive, if one day you intend to sell
- Can be heavy
- The zoom ring is at the front while the focus ring is closer to the body, kind of "reversed" from the usual lens. Due to it's short design, you may find yourself "accidentally" touching the focus ring while zooming. Not a problem for me. If you are not someone who will accidentally hit your mode dial and changed shooting modes, you shouldn't find this an "issue"
- Aperture changes while zooming
- Extending barrel while zooming
Hope the above input will be useful