I wouldn't mind the weight of the lens, but how do you think does the 70-300 compare image wise against the 100-400?
Sample shot both at 400mm to help you decide. I had to aggressively crop them due to file size limit.
Both were handheld. The Hydrant was 18 meters away and the weed about 2 meters or less away. Both f/5.6 shutter was 1/320 and 1/400. It was a bit windy. High ISO's were used due to available lighting... (cloudy day)