I am surprised how close in size they are. The X3 has grown through the years? I would think the X3 is going to drive nicer but who knows. Try to get a drive in both and pick what you like. FWIW my kids all love driving the XC60 because it is easy to see and drive.
The X3 certainly grew in size compared to the previous gen (E83) which is one of the reasons I'm looking at the F25 and particularly LCI (2015-2017). I don't think there is a significant difference in size between F25 and G01 (2018+), at least not visually, but I might be wrong.
The F25 is just the right size from my perspective. The XC60 is also right sized, but to my eyes, the cargo area seems a bit smaller.
BTW - do the rear seats slide and/or recline in the XC60?