When you say "midrange" lens, I assume you're aware that 28mm, with a 1.6 sensor is no longer wide angle, but closer to a "normal" lens on a 35mm film, or FF sensor.
The Canon 17-85 S IS would give greater range, and paired with a 70-300 would make a nice two lens system. ( I like to travel light, and am thinking of shelving my 100-400 L IS, for a 70-300 DO.
The Sigma 18-125 seems to be well received, albeit a bit sluggish.
I've always had great results with Tamron lenses, and wouldn't hesitate to buy any of their DI lenses.
Leigh