The Canon 17-85 is not an f2.8 it's f4-f5.6. This is a problem for video because your exposure will change throughout the zoom range when wide open.
The L series 24-105 has an image stabiliser but is only f4. The IS feature is useful if you need to do a lot of shooting off the tripod. This focal length range is enough to cover the "portrait" end for interviews and still give you an adequate wide angle. This is the standard lens I leave on my camera in case I have to grab a quick unexpected shot.
Unsure why the 24-70 you tried looked soft, it's normally a highly regarded lens. Was it the older first version? A second improved version was released and now they're going into a Series II version which is quite a bit more expensive.
Cinematographer, Steadicam Operator, Final Cut Pro Post Production.
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