Do you have any other units ? 'Cause even 3* may be worth upgrading if you're severly lacking a specific type of units and if they have good Boon/Bane, if you're not against some efforts.
Between the units you showed, considering your 5* units, I'd rather say Hawkeye, since you lack of green and physical units. That, or Sharena. The problem is that he's rather meh but fills a weakness in your team. Usually I prefer to have a green Magical unit, but you already have too much of them.