One almost compares apples to oranges when putting the Lancaster up against the B-17. They were both very good at their intended roles, although I have to say I'd lean more towards the Lanc as the champion as it delivered a much heavier bombload (surprisingly accurately with the target marking techniques later in the war).
Easy to say the Lanc wouldn't survive quite as well in daylight as the B-17, but that's not what the Lanc was designed for, so its a moot point. Likewise, the B-17 was overarmed and armoured for flying at night.
As a closer, I visited the Canadian Warplane Heritage Museum on the weekend and saw one of the world's two flying Lancasters (huzzah!), very beautiful and awe-inspiring. Even my "non-partisan" girlfriend who knows little about planes besides which end is the front and back decided the Lanc was her favourite plane of the museum (woohoo!)
-- Chris