I had the same dilemma for my wife.
Just a simple comparison - if you sit in both, X3 feels more like a big scale hatch, Q5 felt more like truck. My wife really wanted sedan, but we have 2 kids and the size of X3 is perfect.
When I compared 335i xDrive to S4 I ended up getting S4 which I will pick up this saturday, so I don't really have strong brand preference, I just go with the best choice at any given point so X3 and S4 were my best choices.
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