Tamron 24-70 f/2.8 or Canon 24-105 f/4
f/2.8 won't do that much more than f/4 in low light. The midrange is blessed with lots of fast prime choices. I'd suggest choosing between the Tamron 24-70 and Canon 24-105 based on how you'd use it outdoors and travel. I'd then couple it with a fast 35/5...