I'm using the 70-200 f2.8 myself.
Note that f2.8 will get you a shallow DOF, even if you are taking landscape shots (I'm using it for portraits only). So this would typically not be advantageous.
If you are taking landscape shots, you should be using a tripod. If you are using a tripod, you should not use Image Stabilizer (as per my Canon Camera manual). If your camera is steady, the IS may introduce vibration and spoil your picture.
If you are not using tripod, IS will help if your exposure time is long. See the test reports on which focal length & exposure time combination the IS helps. From a certain speed onwards, pictures will not be blur even without IS.
If you are in low light, IS will also help. For F/4 and IS will be quite similar to F2.8/non-IS. The advantage of F4 would be the greater DOF, unless you just want portraits. Then F/2.8 would still be better.
Just to share my thoughts.