I want an L lens and can afford either the 70-300 or the 100-400. Which one?

70-300L

Very sharp

lighter, very compact.

superb IS.

Good 'reach' on a 7D and can be cropped a bit more but still wont equal the 100-400.

Latest technology.

100-400L

Better reach despite being slightly less sharp.

Much poorer IS.

Bulkier and heavier.

Rumoured to be updated imminently-new 100-400 may be ideal but likely to cost a lot more.

Difficult decision. If you're usually in fairly good light, shoot fast moving subjects and tend to be reach limited I'd go 100-400. If reach is not the over-riding factor and you often shoot slower moving subjects in lower light the 70-300L may be better because the IS is superb.