The Sig is a better pistol all around, but this is like comparing apples and oranges.
The Sig is "old school", with a hammer, sear, safety lever, and may be more intuitive for your dad if he is an experienced shooter but is not current on designs and trends.
The XD is a striker-fired pistol, so the trigger is squishy and long, but is the same for every shot, like a Glock. I had a customer, very indignant and upset with his new XDm, rant for about 5 minutes that the trigger didn't seem all that much better after he had paid $140 for a trigger kit and installation. He said the trigger on his 1911 (which he had traded for the XDm) was better. When I explained the dynamics of striker-fired guns, he got a disgusted look on his face and asked "then what the hell good is it?" And you know, I didn't have an answer for him.