they are the most similar to each other in terms of kit, better off getting someone who fills another role or counters an enemy pick.
Widow takes a while to get her Ult while Hanzo always has that wallhack right at the start. I find his ability very useful. It's also easier for Widow to attack the turrets than Hanzo, imo. I dunno, I see nothing wrong with them at all. If the players who are using them ain't doing a good job, I'd switch over to a Widow myself to take out the turrets.