This thread concerns the best medium bomber and not the most effective single 4,000lb piece of ordinance that could be dropped.
As between the Mosquito and Invader, both with 6,000lb bomb loads, I believe the A-26 is the better aircraft. It could take more punishment, had radial engines and a significant defensive armament.
As to flight performance, I don't know how the Mosquito handled with 6,000lbs of ordinance but have heard that she was slow, sluggish and unforgiving. I believe the A-26 performed better with a heavy ordinance load.
Perhaps someone can produce some stats for us.